Take a screenshot
Flick from left to right in the gesture area. The screen flashes and you hear a camera shutter.
The picture is saved as a PNG file. It's in the Screen captures album in Photos & Videos, and in the screencaptures folder when you copy files to your computer (learn how).
Checked against the LuneOS code and installed apps. Nobody has tried these steps by hand yet, so tell us if something differs.