Vaulted Ceiling Trim Molding

The best way to do it is to make a small transition piece of crown molding that acts like the crown is being installed on a flat ceiling on one side of the piece but on the other edge it is making a transition as if the crown molding is climbing a stairway making the bend that happens as you transition from the sloped ceiling along the stairs to the flat ceiling below or at a flat landing.
Vaulted ceiling trim molding.
Source : pinterest.com