extract to steamapps/username/counter-strike-source/cstrike/
To choose the different skin variants, open up pist_usp from your v_models folder and extract the skin you want in that folder.
Old model, but still a good one ;)
If there is anything wrong with the credits or submission, please let me know so I can fix it asap. It doesn't include a world model, but a world reskin. So make sure you get rid of any world models before you use this.