Click Create New Disk, and you’ll be asked a few questions. The most important step is the hard drive size—the default is 64 GB, which is plenty, but consider allocating more space if you plan on installing multiple large games. You can also choose the drive location, if you like, and set a password for opening SteamOS.
Click Create, and the application will download SteamOS and install it in a virtual machine. I recommend using this time to pair a game controller with your Mac in System Settings under Bluetooth. FX Steam Launcher works with most current gamepads, including Xbox, DualSense, DualShock, and Made for iPhone. I used a Nintendo Pro Controller.
Eventually, you will see the login screen, where you can enter your Steam credentials. Once you’re in, you’ll see your Steam library. The virtual machine operates like an actual Steam Deck, which takes some getting used to if you’ve never used one. It divides your library into games known to work in SteamOS and those that might work, which is helpful. A lot of my games started running immediately, with full gamepad support.
Courtesy of Justin Pot
This isn’t a drop-in solution for every game. SteamOS itself is quite fiddly, and you’re now running it in a virtual machine. In some cases, I needed to tweak the graphics settings for games to run correctly. For Psychonauts, specifically, I had to download a user-made patch to get the controller working properly—but I’d have needed that fix for an actual Steam Deck too. When a game ran poorly, I frequently found the solution by Googling and finding actual Steam Deck owners who ran into similar problems.