To install simply extract the "pist_deagle" folder to this location : 

C:\Program Files\Valve\Steam\SteamApps\>YOURNAME<\counter-strike source\cstrike\materials\models\weapons\v_models

Thats it. Enjoy.

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Credits

Model - Valve
Skin - Conorjd

For any major issues with the skin please PM me on www.fpsbanana.com. Thanks.