Swarm SDK Create a Mod

Addons

You must have Alien Swarm installed on Steam.
You must have Visual Studio 2005 installed (or see Compiler Choices for other versions).

Launch the Alien Swarm - SDK from the Steam Tools section.
Double click Create a Mod
Enter a name for your mod. This is the name of the mod folder that will appear in your C:\Program Files (x86)\Steam\steamapps\sourcemods\ or C:\Program Files\Steam\steamapps\sourcemods\ folder.
Enter a location for the source code. The Alien Swarm source code will be copied to this location where you can modify it and build it.

Follow the steps given after completing the Mod Creation Wizard: Swarm Mod wizard complete