All you need to do is have the crown molding sitting in the mitre box (or a jig, if necessary) at the angle it will sit when it's installed.
In an example case, you might have the ceiling-side butt up against the back wall of the mitre box and the wall/cabinet side butt up to the bottom of the mitre box.
Then simply cut a 45 degree angle.
Be sure to plan out which side of the molding you'll cut and how the molding should be oriented (e.g., "cove-side up or cove-side down") and write it down for reference to save yourself countless frustrations and several trips back to the lumber yard for more moldings.
I found that constructing a compound mitre jig works well for a table saw (and may or may not be a little more difficult for a radial arm saw) -- but if the molding is small enough and you don't mind, a simple mitre box and a nice sharp saw do the trick just fine and without all that extra time making a jig!