Andy in Vegas, let me take it more step by step. Cut the 45 degree angle with the crown up side down in the miter saw. You want to have the crown part that will be on the ceiling against the bed of the saw and the part that will be on the wall against the back fence of the saw. You want the crown to be flush on both surfaces so it exactly replicates the position it will be in with these two surfaces. You can clamp a scrap of wood to the saw bed in front of the crown to help hold it in position if you need to. Set the saw at 45 degrees with the longest part being on the bed of the saw and make the cut.
Now take the crown out of the saw and use a pencil to darken a line where the face of the crown meets the fresh cut. Get your coping saw and cut along the drkened line. Don't hold the coping saw at 90 degrees to the face of the crown when you make this cut, hold it at about 5 degrees more so the coped cut leaves a little bit of a pointed edge when you saw along the darkened line. This is hard to explain but easy to see when you are doing it.
Now test fit the crown. Use a pencil to mark any areas that don't fit perfectly. Take the piece down and sand it lightly in just those areas. Since you have the point, you will not have to sand much to get the joint to be perfect.
If you have a foot long piece of crown that you can practice on first, try this. I think that you will be surprised how easy that it is. Or at least I hope that the written instructions will come together for you when you do it.