To start with, you need common sense

To make the process easier for you:
Click on the link to the skin you want to use (i took grunt as an example).
It sends me to that resource here on the website.
http://www.hiveworkshop.com/forums/skins-552/grunt-blp-114013/
Then you just scroll down and see this:
Path & Unit
Import the skin to the following path:
Units\Orc\Grunt\Grunt.blp
The skin will replace the following unit/doodad/texture/interface:
Grunt
That means that you should import the grunt skin into the editor and change its path to the one mentioned above. Restart the editor, and you will see the custom skin in the editor. (you do not need to restart the editor for it to work ingame!)
I know this is a Stupid Question but, Who are the Mag'har Orcs? My lore is in low supply.
That is not a stupid question.
I do not know much about them myself.. But becouse i have played wow.. I know some stuff about them.
What i know: They live in outlands, Hellfire Peninsula (if that is the exact name). And.. that is all i can remember actually..
I did read about them a while before, but i cant remember, you can search for it though
