I can't tell you how many thousands of feet of crown I've put up over the years. The EASIEST way to do crown is to cut yourself sample blocks for the corners, both right and left sides, and if you're doing inside and outside corners, make sure you have a set of blocks for each. Make sure you mark them "left" and "right". I'm going to tweak your brain a little bit here, when you mark the blocks, your instinct is going to tell you that as you're holding them up in the corner the one in your left hand will be the "left" block, likewise for the right. That would be wrong because when you're working with a stick of molding, left and right become reversed.
Once you've got the blocks, there's no guessing about how to set the saw, you just set the sample block on the saw and set the saw to it. Easy-peezy!
When it comes time for cutting, ALWAYS measure your long points. Cut the line exactly or just a female pubic hair over, because it's easier to trim a touch more than it is to find a lumber stretcher to lengthen that piece you just whacked short.
You can custom fit the corners using your sample blocks to find the perfect angle, or just set the miter at 45 degrees and cut it then caulk the joint when you're done. You'll be caulking the joints when you're finished anyway, but you still want as tight a joint as possible.
Another way to do it is to cut one side so that it butts into the adjoining wall, then cope cut the mate. To do this you cut the angle, then hand cut it with a coping saw following the pattern cut into the face with 3 to 5 degrees of back bevel so that the face butts tightly to the one that ran long. ALWAYS cut the cope first before cutting the length so that you can get it the way you want it - it may take several tries. Once the cope fits properly, then measure and cut the length to butt into the adjoining wall.
Third option - call a trim guy, he'll have it done in about in about the same amount of time it took you to read this.
