Measure the placement of the mobile and make a mark with a pencil.
Drill a hole in the ceiling using a 5/16" drill bit.
Insert a #10 drywall anchor into the hole. Tap the anchor with a hammer until it is flush with the ceiling. Then insert and screw in a #10 hook screw.
If necessary, mount the mobile according to the manufacturer's instructions.
Hang the mobile from the hook..