1. First of all, create an account. This will let you log in both the game and the site. Click here to register.
2. Setup your WoW client either by:
a. Downloading WoW 1.12.1 from here, and running 'Reforged Launcher.exe';
or in the case you already have 1.12.1,
b. Downloading our launcher here, copying it into your WoW folder, then running 'Reforged Launcher.exe' (the launcher will automatically update your game client and realmlist);
c. Updating manually without the launcher, by downloading our patch from here. Extract it to your WoW folder and overwrite to all. Open your realmlist.wtf file in notepad, and set the realmlist to 'set realmlist wowreforged.ddns.net' without the quotation marks. This way you won't receive updates and will have to download our patch every time we release a new one.
3. Log in and play!
If you're using a macintosh, here is a setup guide for that: link
If you need any help, do not hesitate to submit a support ticket.