Just the same old!

Place the pist_deagle folder into valve/steam/steamapps/<your_username>/counterstrikesource/cstrike/materials/models/weapons/v_models

Place the w_pist_deagle folder into valve/steam/steamapps/<your_username>/counterstrikesource/cstrike/materials/models/weapons/w_models

And There You Have It!

By Kermitthefrog62