I do alot of compound mitre cuts when doing ornamental plasterwork.
My problem is that cornices that I usually do are quite large. Sometimes being three feet high or larger.
There are no compound mitering saws that you can buy (at the local hardware store) for doing this large of a cornice. So I have to use antique methods and techniques.
This method may be a bit difficult for me to explain here in this type of forum. but I'll try. Please bear with me on this.
Use an angle gauge on the area where you'll be installing the cornice work. you'll need the angles for the wall and ceiling juncture. You'll also need the angle for the corner juncture.
Take these angle measurements and transfer them to a sheet of plywood.
Use your angle tools to first draw the angle of the corner. You'll end up with an inverted V on the wood. This represents your corner.
Find the center line in your V and draw a line out. So you'll end up with a V and a | (line) between the V.
Then cut another piece of plywood (I'll call it plywood #2) that is a little wider then the width of the cornice you want to cut (not the length). It should be about five or six inches wider and the width of the cornicepiece. For the length you'll need to have at least three or four feet.
This second piece of plywood (plywood #2) will represent your walls.
Take plywood #2 and, using your angle gauge, attach it to one of the outside lines in your V you have on the plywood "ceiling". Be sure the edge of the #2 plywood is right up to the center line between the V and it is standing up with the same angle that is in your walls to your ceiling.
What you're trying to do is reproduce your wall and ceiling on the floor with the plywood.
Now when you have the plywood #2 sticking straight up (with the appropraite angle) from the V line You put the cornice piece up to the edge of the #2 plywood and cut the cornice along the edge, following the edge of the #2 plywood and the line you place between the V.
Once you get experimenting with this method you'll find it very effective. The old-timers really knew their stuff!
If I get enough requests I'll make a photo series for showing how to do this.