Checklist:

-- Check you have patched to the right version of Warcraft, many emulated servers run on patch 1.5, or 1.6 due to the time it takes to get the code.
-- Find out the server address of the realm you wish to access, this can usually be done on the front page of any of the servers websites
-- Register, and be accepted - so that you have an account to log on to.

To access the realm, this what you do:

-- Go to your WoW directory and open the file named "realmlist.wtf" with MS Notepad.
-- it will either say "set realmlist eu.logon.worldofwarcraft.com"
or "set realmlist us.logon.worldofwarcraft.com" depending on which version of the game you're playing.
-- Next change the "...us.logon.worldofwarcraft.com" with your desired server, so that it may read something like this::

"set realmlist 13.134.121:1341" (fabricated, do not try and access this)

and that's it - you're good to go. By law this IS legal - you've payed for the game, you're allowed to play it where you want. However it is highly illegal to obtain and use the source code (so dont host em)..

-- Have Fun