Typically you install a crown molding at the wall to ceiling. That is placed at an angle and cut at a compound angle.

I have seen flat stock placed there too though...

At any rate, whichever you are doing flat or compound crown I do have a step by step tutorial that explains how to do both.

As for the protractor best for either project you'll find the links to the starrett "prosite" on the www.installcrown.com site which we use extensively. With it you will place the protractor and it will give you the miter cut make it a fairly simple process.

Alter Eagle Construction & Design

http://www.altereagle.com/ | Construction & Design | http://decks-ca.com/ | Decks, California outdoor living | http://kingofcrown.com/ | Molding and finishing | http://installcrown.com/ | Crown tutorial