- Close the game if it's running
- In your Steam game Library, right click on the game, and right click on Properties
- Click on the "Launch Options" button on the first tab.
In the Launch Options dialog, enter this string (no spaces):
When you start the game, it should default to full-screen, non-windowed. It will also remember this resolution after you quit, so you can remove the -fullscreen Launch option if you want to run in windowed mode instead.