go to store and purchase the pre mitered corners- either use them or place them on your miter saw to mark correct angle(s)

Even though your miter saw cuts one way you can still cut both ends. It depends on how you position your piece on the saw. If you are using a non-compound saw, some corners will be cut with the bottom resting on the saw body and others will be resting on the fence.
Always place molding on saw as it sits on a wall, sometimes piece will be upside down.

Bob Vila: how to for crown molding http://www.bobvila.com/articles/bob-vila-radio-crown-molding/

Good luck
David
writer@ www.fixupthishouse.com