1- Download the add-on
2-Open the zip-folder and copy whatever you find inside
3-Go to program files > wow > interface > add-ons and paste the file in there
4-Restart WOW.
5-Now in login screen (where you see your characters) you will see on the bottom left corner a button 'Add-ons'... Click it and inside you can enable all the add-ons you want.
6-Finally press 'Enter World' as usual to get in the game !
ENJOY !
(Nnahotrod here): i know steel used HOURS on writing this but i use 5 min on a movie so here it is:
https://www.youtube.com/watch?v=ayFwEv6PTpY