Prerequisites

Steam

Source SDK 2013 Base Singleplayer (install via the tools page on your Steam library)

Installation

Step 1: Download the game on the downloads page.

Step 2: Copy and paste the game into STEAMDIR\steamapps\sourcemods. If the folder doesn't exist, create it yourself.

Step 3: Restart Steam.

Shoot Stuff should now be in your steam library. Enjoy.