First, start game as usual way from steam, setup settings and play some match.
After:
1. Download and unpack https://drive.google.com/file/d/0B0wEG3EQk0z5OFdwVkYzdDlnMnc/view?usp=sharing&resourcekey=0-ZJumMyrUVnDGG3G6o9pm6Q (Need .Net Framework 4 client profile for working https://www.microsoft.com/en-us/download/details.aspx?id=17113)
2. Start NucleusCoop.exe
3. Press "Search Game" or "Auto Search" to add "Alien Swarm Reactive Drop" to Games List (usual reactivedrop.exe place: c:\program files (x86)\Steam\SteamApps\common\Alien Swarm Reactive Drop\reactivedrop.exe)

4. Choose "Alien Swarm Reactive Drop"
5. Setup keyboard and gamepads places on the screen

6. Click > and > again
7. Choose map (deathmatch maps are at the bottom)
8. Click PLAY
9. Wait for a minute about (all apps will start and placed to the corners)
10. Gamepad players can choose their marines by pressing Start button
11. Mouse player can choose weapons for all squad, setup difficulty and other settings during briefing and press Ready to start mission
12. Enjoy

Deathmatch maps are supported also. Gamepad player can press Back button to show/close "Choose marine" panel and Start button to choose marine.
Credits: NucleusCoop tool: https://github.com/lucasassislar/nucleuscoop
Controller buttons description: https://steamhost.cn/steamcommunity_com/app/563560/discussions/0/144512942755065884/

Also added 2 custom campaigns maps to maplist:
Dead City https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=909702841
City 17 https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=861497042
If you wanna play other custom map - just type in console "changelevel mapname"