Measure the distance from the ceiling to the desired top of the mobile with a ruler. Ensure the mobile is high enough so no one will walk into it, by adding 4 inches.
Cut a piece of gray thread to the measurement. Fold over one end of the thread 2 inches. Make a circle with the folded thread and poke the folded end in the circle. Pull the thread tight to make a barrel knot loop.
Tie the opposite end of the thread to the mobile with a secure double knot.
Add a drop of white glue to the barrel and double knot with the toothpick. Let the glue dry for 20 minutes.
Screw the cup hook into the ceiling. Hook the barrel knot thread loop over the cup hook.