I use glue and corner clamps, then use an air nailer to put a nail in the corners. I have my kids at school make frames this way, as long as they measure the frames come out good.
The last time I made a frame at home however, there was a lot of swearing and I took the whole piece right out to the fire pit. The miter saw I used had a crappy blade and angles were all messed up.
I sometimes use messed up student frames in my projects. I cut the angles to 45 and adjust the sizes to what ever fits the kids screw up, then fit my project to that size. If you think you might have some trial and error, you might want to keep the pieces.