1. Unpack and copy all of the folders (materials and models) to the above mentioned location, paste and press yes on each pop-up.
2. Install Notepad++ (http://notepad-plus-plus.org/) and open the 'pak01_dir.vpk' file located in the same earlier mentioned directory. MAKE A BACKUP FIRST!
3. When it opens, go to models -> player and there you'll find plenty of 'ctm_' and 'tm_' skins such as ctm_fbi (and variants) or tm_leet (and variants).
4. Press CTRL+H and a window will pop-up. There you'll have the option to change a certain work with another in one instance.
5. In order to make your selected skins work in-game you need to rename at least 1 letter/character of the name of those skins, however DON'T add extra ones. Just replace an A with a D or something. Example below:
Provided you want to change ctm_fbi to the skin you downloaded, you need to rename ctm_fbi to something like ctm_fbc. I changed the 'i' with 'c' and now when you open
CSGO it won't be able to read the default one anymore. It will instead read the custom one that you downloaded and put in the csgo/ folder.
You CAN'T however do something like ctm_fbiefazfazefafaefe or ctm_fbi0 as that will prevent CS:GO from opening.
6. In the "Find what" area you insert 'ctm_fbi' and at the "Replace with" area you insert 'ctm_fbc'. Press 'Replace All'
7. Once done for any or all of the skins, press save.
8. Enjoy your new skin!