Safely work above stairs: Open a step ladder and place it at the top of the stairs. Then position an extension ladder on the stairs, leaning it against the opposing wall (pad the arms to protect wall paint). Construct a platform to span the distance between the ladders by screwing 2x4s to a plywood deck. Nail stops to the ends of the 2x4s to prevent the assembly from sliding off the ladder rungs. thisoldhouse.com
Cut crown molding: Place molding upside down so the back of the bevel meant to make contact with the ceiling rests on saw deck. Secure with clamps. Cut strip of plywood 30" long to be horizontal fence. Hot glue the saw deck on both sides of its center. Press the fence tight to the crown. Remove crown; cut center section of the horizontal fence at 45 degrees in each direction. Nestle the crown between the saw's vertical fence and your horizontal one to securely hold it in place for mitering.