Open Up "My Computer"
Goto To You're Steam Allocated Files.
Which Ussualy is "Local Disk c"
Then Goto "Program Files(x86)" if not there just "Program Files" then.
The Find Steam Folder "Steam" Duh!!!
Then Find The folder Named "steamapps"
After Opening it Click on the folder named "Your Steam Usser Name" this is not the actual name there should be your steam nick or the name You Registered to steam with.
Then open up The folder Called "counter-strike source" & after that folder "cstrike"
This is the tricky part if you never had skins these folders might not be there So you will have to create Them.
 Folder in "Cstrike" Folder as Folows:

"Materials"*->"Models"*->"Weapons"*->"v_models" & "w_models" you need to make both folders if their not ther after making the folders just put it in as folows The folders in The Winrar File.