Hiya! Because my Thread was bugged I reposted my own one. I will show you how to add Skins into your CS:S!
Step 1: First of all a site for Skins ofcourse:
Step 2: Now you will see different Skins! Press on a Skin you like and press the Download button. (In the right upper corner you will see different Categories like AK47, P90, MP5, M4A1 SIG-552, etc.)
Step 3: Now when you downloaded the Skin there should pop up a WinRAR window. With the Skin file now open your CS:S directory (in my case C:\Users\MEDION\PC\Desktop\CounterStrikeSource2010 \) and open your cstrike folder.
Step 4: You will see Materials, Models, Scripts, and Sounds (depends from Skin to Skin).
Step 5: Now copy the Materials into the Materials folder (the materials folder from WinRar into your Materials folder from cstrike)
Step 6: Now do the same for Sounds, Scripts and Models.