Well, say your skinning G-Man. You go to the Gmod9 materials folder, then go to the models folder in that. You then see if there is a G-Man folder in there. If there is, great. If not, make a folder called "G-Man".
Then you paste the original VMT of the thing you are posting(which you probaly got when you extracted the character's folder) and your new VTF into the folder named after your character.
Then, you simply spawn "G-Man" in game, and there you have it!
Any questions?
Oh, and when i say G-man, it could mean any character that you have skinned.