1. Create an account
Register for free on this website. Your username and password are the same in game.

2. Get a 2.4.3 client
You need a World of Warcraft: The Burning Crusade 2.4.3 (build 8606) client.

3. Set your realmlist
Open realmlist.wtf in your WoW folder (or Data/enUS/realmlist.wtf). Replace everything in it with:

set realmlist logon.cavernswow.org

4. Log in
Start Wow.exe (not the launcher), log in and create your character. You start at level 255 in the Caverns mall.