First of all you'll need Steam, which you can download here: Store.steampowered.com . After you install, you'll be prompted to create a free account.
Then you'll need to install Alien Swarm: Store.steampowered.com
Finally, you'll need to install the Alien Swarm - SDK. The link here contains instructions: Developer.valvesoftware.com
Enjoy!
I have Alien Swarm and Alien Swarm SDK installed, but do I put this mod in a Sourcemods folder or do I put it in the Alien Swarm directory? And more importantly, will I get VAC-Banned if I do the latter?