Download Btex addon, then download the textures you want, unrar them into ...wow\interface\addons\btex\skins then start up WoW.

In WoW type /btex menu, then click the scroll-list and select "Custom texture" - at this point a new window will appear and in the texture name field you enter the name of the Btex texture you want.

The name of each texture is what the .tga files that you copied into your skins folder are named, minus the "-#.tga".

So in this case, the files are named "RexEngineer-1.tga" and goes up to -4.tga, so you just need to enter "RexEngineer" into the texture name field.