1) Copy the models, sound, and material folders into steamapps/common/Counter-Strike Global Offensive/csgo/
2) Make a back-up copy of the PAK01_DIR.VPK (just in case)
3) Using Notepad++, open pak01_dir.vpk (Notepadd++ is a free source code editor. DO NOT USE REGULAR NOTEPAD OR YOU COULD MESS UP YOUR PAK01_DIR.VPK!)
4) Press Control+H and replace v_smg_mp7 with v_smg_mp8 (use the "Replace All" feature. It should tell you that 4 occurrences were replaced)
5) Next, use the "Replace All" feature to replace mp7-1 with mp8-1 (this is for the shoot sound. It should tell you that 2 occurrences were replaced. You may have to scroll back to the top of the document if it doesn't work)
6) Save the file

To uninstall, restore your original pak01_dir.vpk. If you have other custom skins installed, simply use the "Replace All" feature to rename v_smg_mp8 and mp8-1 back to v_smg_mp7 and mp7-1, respectively. Deleting the models, materials, and sound folders isn't necessary.

Enjoy!

-DMG