1. Backup (really; Backup! Backup! Backup! Backup!) your existing WTF and Interface folders in your WoW directory, then remove them.
2. Unzip/unrar and place the containing Fonts, Interface and WTF folders directly in your WoW directory so it looks like this.
\World of Warcraft\Interface
\World of Warcraft\WTF
\World of Warcraft\Fonts
*** Remember to do the following steps for EACH account/character you want to have this addon-pack installed for ***
3. Open the WTF -> Account folders, and rename the "Account name" folder to your own account's name.
4. Open the "Account name" folder you just renamed, and rename the "Server name" folder inside to correspond to your server's name.
5. Open the "Server name" folder you just renamed, and rename the "Character name" folder inside to correspond to your character's name.
6. Launch the game.
7. When in the game, it will look like a total mess. Don't worry, it's fixed with one simple command!
/reflux switch DevlinUI
*** Please note that this is CaSe-sEnSiTiVe! ***
9. Enjoy the game!