Basically you open up the War3Local.mpq (This is the one you will be replacing). Then you put all your files in there, make it back into an mpq, and replace your War3local.mpq (Remember to back up)
Then whatever folders you put the files in, in the mpq, that will be the path to access it in the editor/game.
The one thing you will need to watch out for, is when you put textures in the MPQ, make sure you use the right path or the models won't be able to find the textures.
There is an application called MDX Pather, that will allow you to view the paths of the textures that a model uses, and change them if necessary.