Once the feature is rolled out to everyone, you can turn it on by pressing and holding the window handle at the top of an app’s screen. If you have a keyboard attached, you can also use the shortcut meta key (Windows, Command, or Search) + Ctrl + Down to activate desktop mode. (You can exit the mode by closing all your active apps or by dragging a window and dragging it to the top of your screen.)
Google notes that apps locked to portrait orientation are still resizable, which might make things look a bit weird if certain apps aren’t optimized. However, Google plans to address this in a future update by scaling the UI of non-resizable apps while maintaining their aspect ratio.