Easy. Fast. Easy... Wait, didn't I already sa that?
How to add addons to Garry's Mod 9, 10, and 11:
1. Find the skin you wish to download.
2. Download it and wait for it to open*
3. Open "My Computer" on Windows ME, 2000, XP, 98, Vista, or 7, or "Finder" in Macintosh, and Search "garrysmod"**
4. Open the "garrysmod"*** folder that should pop up
5. Whatever person that tells you to put it in add-ons is dead wrong, inside the archive*, extract* the contents to the garrysmod folder. Remember: NOT THE ADDONS FOLDER. Open the "addons" folder in the archive*, and extract THAT into your "garrysmod" folder. If it asks you to replace the crap, press "Yes to all" in the dialogue box that should pop-up. It won't replace anything you have, but it will extract the contents of that folder into "garrysmod"*** folder and it should work.
6. Start up the game and whatever crap you just downloaded will be in there. Remember: DRAG-n-DROP!!!
*You will need WinRAR archiver, 7ZIP, or WinZIP to open it, unless it is an exe.
**Optional. If you know where your garrysmod directory is you can move on to step 5.
***THE GOD---N SECOND GARRYSMOD FOLDER!!!!!!