There are a lot of threads out there telling people how to apply to a guild but, as far as I can tell, very few of them come from the point of view of the recruitment officer. So I thought I'd throw in my 2cents.
Step 1:
Find out what your current guild's reputation is.
Wait, what? Why would this affect me? I'm leaving. Well, it makes a huge difference. If your current guild has a reputation for being bad or douchebags, you will have to work to convince the guild you're applying to that you're not one of those people. Nobody wants That Guy, and some guilds have a reputation for being That Guy storehouses.
For example, we could have bled a certain similarly progressed guild of at least half their members but the contact I got from them was, on average, less "I can't stand these people" and more "You should be grateful I'm even considering you, you worthless sack of dung. I expect to be made guild leader right away." People from that guild automatically get a black mark against them and I'm wary of considering them no matter how good their experience is. A single incorrect gem or enchant is enough for me to turn them away.
On the other hand, there are certain guilds that have a very good reputation where I'm willing to overlook a lot to get my grubby little paws on one of their members (We have at least one person who is capable of playing the applicant's class/spec extremely well and can teach them). After making sure that they really want to leave of course.
Step 2:
Find a non-officer to chat to.
The easiest way to do this is armoury the guild and pick someone from the highest populated rank. This will usually mean an alt or a member raider (depending on whether or not the guild allows alts). If you've found an officer, move on. A guild with more officers than members is bad news.
When you've found someone suitable, ensure that they're not in an instance and whisper them politely to ask if you can have a minute of their time to chat about their guild as you're thinking of applying. Be patient. Wait 5 minutes and then try again if you don't get a response in that time. If they don't respond on your third attempt, try someone else. If you don't get any responses, even to try redirect you to an officer, then bear in mind that the guild atmosphere is probably very unfriendly to new people.
When you do get someone to chat to, ask how long they've been in guild, what raid atmosphere is like, what guild atmosphere is like, how drama is dealt with, if the officers are nice, etc. Don't ask any "officer" questions (what loot system do you have, what are the raid times, what consumables are provided, what am I expected to bring etc). See if you can start up a nice, fun chat with the person. If they're new, ask them to direct you to a long-time resident of the guild.
Step 3:
Go to the guild website and look around.
Read everything you have access to without signing up. There should be a progress report, a set of guild rules, an application form and, if you're lucky, all applications will be public.
Look at the progress stuff first. If there are kill vids, watch them all. This will give you a good sense of raid atmosphere and the people in the guild.
Read the rules head to toe. Make a note of anything unusual. One guild that I've looked at had "Your favourite flavour of jellybean is Crazy Confetti. Make a note of this, it will be important later." stuffed randomly into their guild rules. I giggled and moved on to their application form and there, right at the end, was the question I was expecting: "What is your favourite flavour of jellybean?"
Read recent applications. Look at the experience that the applicants have and the reasons for a decline or accept. Look at their WotLK experience as well as their current experience. Check the dates on their achievements. If you have the same or less experience than someone who was declined on account of lack of experience then don't waste your time or theirs applying unless you've had a direct invitation to apply from an officer (this does not mean "I whispered them after they posted their ad in trade and they said go apply", this means "I ran in their alt run as a pug and someone asked if I'd be interested in joining them and gave me the website address").
Step 4:
Decide whether or not this guild sounds like a place you would like to be.
Throw progress and moving up in the ranks and so on out of the window. Pretend you are joining a social guild and intend to stay there for life. Is this the place that you want to be for the rest of your life? What are the odds of you staying for longer than a month? 3 months? 6 months? Will you be able to deal with these people for 100% of your leisure time for months on end?
The most important thing here is to be honest with yourself. Don't just say "Yes I want to be in this guild" and ignore the warning flags your brain has raised. If a top progression guild seems like a godawfully miserable place to be, then rather find a less progressed guild where you'll enjoy yourself. A reputation for guild hopping follows you to different realms. A lot of guilds look for references from officers of previous guilds while considering your app.
Step 5:
Check yourself.
Make sure your spec is good. If you don't have a cookie cutter build or you have odd talent or glyph choices, make sure they're good and you have solid reasoning behind them. Make sure every single item of gear you have is gemmed and enchanted with the best enchant that's available to you. A reasonable officer won't be upset if you have haste on blue wrists instead of the expensive Intellect/Agility/Strength maelstrom enchant, but don't count on the person looking at your app being reasonable. Make sure to log out in the spec and correct gear for that spec on every single logout until you get a response to your app. Don't equip any new gear even if you log out for less than a minute to go make the enchant and gems on your alt. Assume that luck will not favour you and that less than a minute between armoury updates will be the exact time the recruitment officer looks at your toon.
Step 6:
Make an application.
This might sound strange but, don't use the application template as an actual template. It will make your application stand out from the rest. The minute I see an app which isn't a direct copy-paste with answers in place of the bracketed info I sit up and pay real attention. Everytime I have to decline an app like this a piece of my soul dies. Fortunately, it's extremely rare that someone who makes that much effort over an app is below our requirements (only 1 so far thank goodness).
Write up your application in a separate place, Word, Notepad, Wordpad, whatever and make sure to hit save occasionally (your av deciding to reboot for you when you're just about to hit Submit is soul-crushing). When you're finished your application, check the time. If you spent less than 2 hours writing your app, you did it wrong. Go back and flesh out your responses.
Make sure that you answer every question that is asked, even if you think that they're stupid. Don't look for a right answer because there isn't one. Just give an honest one. Try make your answer to silly or personality questions original and interesting. If I had a penny for every time I've seen "downing a hard boss" in answer to "What's your most rewarding WoW experience?" I'd have... well 20p, because we've only had 20 apps since we added that question. Be specific. Which boss? Be descriptive. Outline the back story and colour in the frustration and the magic of everything coming together and just working the way it sometimes does. Make a joke out of it. Something. Anything! Just be different.
Add logs to your app, even if they're not asked for. Even if you're a tank. I want to see how much avoidable damage you take, use of survival cooldowns, pot usage, damage done, healing done... The works. And don't just give that one set of logs where you were top of the damage/healing meter either. Link a set of logs where you did farm content and one where you did progress content. I want to see how comfortable and reliable you are in progression content as well as how high you can push those numbers in content you're familiar with.
Step 7:
Wait.
This is the hardest part. Some guilds aren't rigorous about looking at their forums. Don't expect to be contacted in-game. Assume that the response to your application will be on forums. Wait a week for a response on forums before whispering anyone in the guild requesting a response. If a week goes by without a response and you do end up having to whisper someone, try make sure it's an officer (armoury is your friend here again - whisper someone in the top 3 guild ranks for preference) and be polite. "I'm terribly sorry to bother you, I made an application on your guild forums about a week ago and I was wondering what the average response time is." The same rules of "not in an instance or raid" apply here. If you bug me while I'm fighting Omnotron I'm NOT going to answer you. Unless you immediately apologise for disturbing me and say you'll contact me after the raid. Don't ask me what time that is - you should know already, having been on the forums to apply.
Step 8:
Don't argue with the person who responds to your post.
Yes, even if they're stupid. If you're asked about talent, glyph, reforge and gear choices, don't be confrontational. Give reasons why you have those talents. If you have any awkward talents like Body and Soul which can cause clashes with a Discipline priest, show that you're aware of potential clashes and explain the utility that you can provide with Body and Soul. If they ask a question that you can't find a reasonable answer to, go respec/gem/enchant/glyph and respond saying "You made an excellent point and brought something to my attention that I can't believe I missed. My sincerest apologies for making you think I am an idiot. Please give me another chance to prove that I'm not." Ok, you don't have to be a sycophant, but do acknowledge any errors on your part and fix them as soon as possible.
Step 9:
Your trial.
But wait, I'm already in the guild. This isn't part of the application process, is it? Yes, actually. It is. You're still not a member of the guild, you're just being allowed to run errands for the cool kids for a bit to see if you're cool enough to hang with them.
Stay quiet until you've made some friends and are comfortable with the guild atmosphere. Don't establish your presence right off the bat. Don't speak unless spoken to. If a fun, nonsense conversation is going on, join in. Contradictory? Not really. Getting a feel for guild atmosphere is good, making yourself the guy that never speaks to anyone is bad.
If you're assigned a go-to person regarding your class, contact them immediately and ask if they have any suggestions for you. Ask reasons if you don't understand and act on their advice immediately if they have good reasons.
Read the rest of forums. Especially any strats that are up. Read the entirety of the strat thread. There might be a crucial change on page 10 to the main strat posted on page 1. If the strat thread has no answers, contact your officer and ask about those strats. Just because the fight was so easy that it was done on the first day doesn't mean there weren't strat changes on that first day, it just means that nobody bothered to update it.
Be helpful. If someone wants a boost, offer to help when you're done with whatever you're doing now. Don't feel obliged to drop what you're doing to help them. Apologise if you're unable to, eg if you're just running your daily random quickly and afterwards you have to log off to make supper and offer to help them another time. If someone asks for people for a heroic, offer your assistance. Obviously don't put yourself out too much, but be as helpful and friendly as you can be within your time constraints.
Be kind and polite to everyone. You never know when that social member you're being slightly condescending to is actually a long-standing member who is currently on a hiatus due to irl issues and will be back his officer spot by the time your trial comes up for promotion/kick. Or the guild leader's girlfriend. Or one of the longstanding raiders' best friend. You have been warned.
How to convince a guild that they want you even though your experience is dismal:
Find out who the recruitment officer is and speak to them.
Be friendly and polite and a little (not too much) grovelly. Say that you know that your experience is below what they would normally accept and that you're afraid to apply because you're positive that you'll be rejected and ask if there is any way at all that they could maybe possibly give you a trial run in their alt run or something. Make sure that your gear is gemmed and enchanted perfectly and up-to-date with the best website that there is for your class (usually
EJ). Don't be rude or abusive if they say no. Thank them for their time and say that you'll go and get some experience and get back to them.
The odds of you being given a trial run is tiny. Don't run away with the idea that this is a surefire method of getting in with minimal experience. It's a maybe only. And you still have to perform to the quality of their main run as well as be universally liked in the trial run if you get it. Being as good as our alts won't get you spot even if I do let you in. Being better than our alts will give you a good chance. Being as good as or better than our mains will guarantee that your app will be accepted, even if this is your first toon and you've never raided before.
How Not to do it:
1. Try and get out of making a forum app. Act insulted that they are too lazy to inspect you in-game or armoury you.
2. Pick a member of the raid team and critique them in your app, explaining why you're a much better choice than my 25k dps hunter who does everything (I mean everything, gongs on Atra, parasites on Magmaw, freeing drakes on Halfus, adds on Cho'gall - you name the dps responsibility beyond "hit the boss until it dies", he does it), was a founding member of the guild 3.5 years ago and has been my guild leader's best friend for 20 years and how you should replace him.
3. Make sure to let the recruiting officer know how grateful he should be that you've condescended to consider boosting them even though they're far below your capabilities. They might be unaware of this otherwise.
4. Demand a spot in the main raid team immediately.
5. Demand that a secondary group is started if told that they're not taking apps at the time.
6. Say that you got kicked out of your last guild because they were *^#*&@^# retards and list all the reasons why they were *^#&@)(@$ retards. Use every swear word you know at least 3 times.
7. Say that you think the guild rules are stupid. Provide a list of stupidities in the guild rules and make your reasoning as flawed as possible. eg, "teh rool wut sez ur not aloud 2 ninja stuf r DUM cos i r beta then teh n00bz wut r in teh grp n i desreve teh stuf 2 seel moar then wut tey r dserev 2 were".
8. If all else fails, be as obnoxious and unpleasant to everyone you come into contact with from said guild.