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Ok check it. Quick summary for you.
Find the Thief Guildmaster or Guildmistress. It is a YELLOW NPC, ignoring blue NPCs should make things easier for you. Set a macro for All Names, and go into different towns repeatedly hitting the macro.
The hot spots to check are Fel Skara Brae, usually near the tailor shop, Fel Moonglow, usually near the healers' guild, Fel Cove pretty much anywhere since it's so small, and New Haven near the entrance or above the bank. Either Fel or Tram towns are fine, but I tend to check Fel more often.
When you find him/her, get within "touching" distance, and say their name, followed by "join". So if he's named John, it'd be "John join".
In order to join, you must have at least 60 REAL Stealing, and must be at least 48 in-game hours old on THAT char (they might have changed it to being 48 in-game hours AND have had that char for a week, not sure, haven't tested it). If you aren't 60 Stealing, he'll say you must at least be a Journeyman Pickpocket to join (at least I think he says Journeyman), and if you aren't old enough, he'll say you're too young, and to return in a week. Ignore this, because you could be one hour away from being allowed to join, and he'll still tell you to return in a week. This is just his macro, and should be changed because it can be confusing.
A good thing to do when trying to get to 48 in-game hours on your thief, is to just leave him online as much as possible. The best place to do this would be a well-populated bank, such as Luna or New Haven, where there is constant traffic. This traffic should keep you from being disconnected.
When he accepts you, he'll ask for 500 gold. You'll need to drop it on him within about 10 seconds, before the offer expires. Then, when in the guild, say "disguise" near him. Disguises cost 700 gold, and can be used multiple times.
After using a disguise, it'll change your name to some random NPC name. You can keep using it to "scroll through" the names, until you find one you want. There are a ton of names, so finding a certain one each time would be annoying.
It will also allow you to pick your new hair/facial hair. Your new hair will be white, which is a dead giveaway to anyone paying close attention, so use a normal hair dye after going into disguise. You can buy normal hair dye extremely cheap (I think like 30 gold) at an alchemist NPC vendor, like the one past the maze in moonglow.
The disguise lasts 2 in-game hours, and is extremely annoying when waiting for it to go away. If you log off and back on while disguised, your char name on your char list will be the disguised name. This is normal and will fix when it wears off. Odds are, when you use the disguise, your in-game macros will mess up. This is a stupid bug, just set them again.
After your disguise FINALLY wears off, your name changes back, and so does your appearance/macros. And yes, your dyed hair will go back to its original color. I only suggest using disguises when absolutely necessary, because they're pretty annoying, haha.
Hope that answered all of your questions.
P.S. I'm gonna try to get this post stickied in a new thread, to avoid this repeated convo in the future.