Hi, curious. This is pretty simple but remember that you only want to cut miters on outside corners. Inside corners should be coped and there is plenty of info on this on the board.
To measure outside corners, use a bevel gage. Put the frame on one side and the blade on the other side and tighten the screw. Now get a protractor. Borrow one from your kid because the plastic ones they use for 8th grade math are fine. Put the frame on the flat edge of the protractor and the blade at the center mark and then read the angle. That's all there is to it.