And on wowhead! http://www.wowhead.com/guide=1564/how-to-transmogrify-and-what-you-can-and-cannot-do
Transmogrification, also known as transmog, mogging, or xmog (but not transmorg), is a fairly simple process.
Firstly, transmogrification and void storage are not related to one another. Quite a few players have gotten the idea that items must be contained in your void storage space to be mogged, but this is counterproductive. In order to mog an item, you must have both the item whose appearance you're using, and the item whose appearance you're changing with you. If you place an item you want to mog in your void storage, you have merely wasted your deposit fee.
The interface itself is straight-forward. Upon speaking to the etheral transmogrifier (located in Orgrimmar, Stormwind, and both Shrines in the Vale of Eternal Blossoms; city guards can mark your map) a window will open showing your character and several equipment slots. The image of your character is a preview of what your gear will look once mogged, and the equipment slots are where you place the items whose appearances you're going to use. You can only alter the appearance of your currently equipped gear, so make sure that you are wearing the gear you want to change before talking to the etheral.
Once the window is open, simply drag and drop the items whose appearances you want to use into the corresponding slots. This means, yes, you must actually possess the item whose appearance you want to use. Continue until you are satisfied with the result, and then confirm your selections. You will be charged an amount of gold based on the vendor value of the items you are altering, to a minimum of 1g per item. Be warned, items used for transmogrification will be made soulbound, untradeable, and non-refundable by finalizing the process.
If you change your mind, the process can be reverted for free by going back to the transmogrifier with the item equipped, and clicking on the item's box in the mogging window so a yellow arrow appears. Alternatively, it can simply be transmogrified again into another item.
Simple, eh? However, there are some restrictions to keep in mind when heading down to your neighborhood mogger.
- You must meet all requirements to normally equip an item before you can use its appearance for xmog. This includes level, class, reputation, and profession skill requirements.
- Armor types must match. Plate armor can only take on the appearance of another piece of plate armor, mail armor can only take on the appearance of other mail armor, and so forth. This is to maintain a degree of visual distinction among classes.
- Legendary items are not permitted to be mogged to or from. This is to maintain the uniqueness of these rare items.
- Only items with stats beyond weapon damage, armor, and proc/on use effects are eligible for transmogrification.
- Certain items are excluded from xmog on the basis of being inappropriately silly. This is to maintain a degree of dignity in the aesthetic of the game. These items are not marked in any way, but you will receive an error message when attempting to mog them.
- Heirlooms may have their appearances changed through xmog, but cannot be used for their appearances, and will lose their mogged appearances when sent through the mail.
With all of that out of the way, go forth and be fashionable! And whatever you do, don't listen to your stuffed tiger no matter how bad an idea he says this is.
Updates are in bold:
- Fixed a bug which caused heirlooms not to lose their xmog when sent through the mail.
- Weapons may be mogged with similar weapons. The compatible types are: (1h sword, axe, mace), (2h sword, axe, mace), (polearm, staff), and (bow, gun, crossbow). Daggers, fists, wands, shields, and off-hands can only be mogged with items of the same type, and you still must be able to equip both weapons normally.
- Level 60 Warlord and Marshal's PvP gear is ineligible for mogging in order to recognize the Vanilla PvP ranking system. Players with rank 7 or higher from Vanilla or rated battlegrounds may purchase eligible replicas from vendors in Area 52, Netherstorm.
- One-handed weapons cannot take on the appearance of off-hand restricted items. This is because some off-hand weapons have models that are designed specifically for one hand or the other. However, off-hand weapons are allowed to use the appearances of one-handed weapons. Many Main Hand weapons have been converted to one-handed weapons in an undocumented change.*
* Some former MH weapons are bugged and still recognized by the transmogrifier as Main Hands. A workaround has been found: mog the former MH weapon with your 1h weapon, and then try mogging the 1h with the former MH again.