Repair all the cracks in the ceiling with the joint compound and a putty knife. Spray a small coat of primer on each spot. Allow primer to dry. Spread drop cloths.
Mix the popcorn and paint together in the 5-gallon bucket. Amounts vary according to the brand you purchased. Mix with the paint stick to the consistency of peanut butter. It will be thinner than toothpaste.
Connect the hopper to an air compressor. Fill the hopper with the mix. Spray the mix onto the ceiling evenly. You will need to keep moving the hopper unless you release the trigger. If you stop with the trigger on you will get a large lump.