1. Get the client

You'll need a WotLK 3.3.5a (build 12340) client. If you already own a copy of Wrath of the Lich King, you can use those files directly.

Download Client

2. Set your realmlist

Open Data/enUS/realmlist.wtf (or your locale's equivalent) in a text editor and replace its contents with:

set realmlist odinsdarkark.duckdns.org

3. Register an account

Create your account here if you haven't already.

4. Launch and log in

Start Wow.exe, log in with your new account, pick the realm, and create your character!