Before you get started, be sure you have downloaded WowMatrix.
Step 1: Open WowMatrix. You'll see a screen that looks similar to this:
Step 2: Click the 'Get More AddOns' tab at the top. You'll see the list of WoW AddOns that you can install using WowMatrix:
Step 3: Select a WoW AddOn that you'd like to install from the list, then click the 'Install' button:
Heads Up! If your World of Warcraft game is already open, you will need to close and re-open the game before it recognizes any new WoW AddOns you have installed.