Well you import the skin (the .blp) via import manager
Add the path for the skin as this is very important (you can either find the path if the page for the skin you downloaded says it, ask us here or search for it through WC3 Viewer's list)
Its done. The skin will automatically apply to its model. You don't need to do anything else now. If you have mulitple skins for one model, you can't do it by just importing more of the same kind but there is a tutorial on how to do it.
Remember in the WE the skin may seem it hasn't applied its effects to the model but it should work in the game.