Instructions
Right-click on the Steam icon in your taskbar and click on "Library."
Right-click on "Counter-Strike: Source" in the list of games which appears in the left-hand pane and select "Properties."
Click on the "Set launch options" button. You can now type any advanced settings command in that window. Click on "OK" to confirm those new settings. "Counter-Strike: Source" now uses those new launch settings every time you start the game.