I measure the room first. The plan for every cope to be the same way. You can either cut the moulding laying flat on the saw, or up on the angle it will be on the ceiling. I have done it both ways. You need the right compound miter to do it laying flat, and then you need to know the settings for the angles.
I mark each piece, and cut them one after another. I then cope them all back. Once coped, I cut the straight end to the proper length. Doing them all at once, and doing them right (hence the practice) saved me a huge amount of time.