To start, you have to find a skin you like! Since you all love the god damn Deagle so much, I'll use that for an example.
I would suggest using models from Counter Strike: Source, since that is what TTT uses for its guns by default.
I will use an example from:
gamebanana.com/css/skinsSo I like this one.
http://gamebanana.com/css/skins/18803 After you download, unzip the materials and models folder. You will not need the sound, scripts, LUA, readme.txt, etc.
You will unzip the two folders to your garrysmod/garrysmod folder. When it asks to merge, you want to select "Yes".
Since these ones are CS:S skins, they will work for you wherever a CS:S Deagle appears in Garrys Mod (such as TTT). This is all you have to do for that.
Most, but not all, simple retexture skins will not have a models folder. This is ok.
How To Skin