My sister in law just completed all the ceilings in her house this same way. She purchased a 5 gallon bucket of ready made drywall compound, and mixed it well. Using a 6" wide putty knife, spread it on the ceiling about 1/8th to 1/4" thick and allow it to nearly dry. Then give it a knock down on the high spots, until it blends with the old ceiling. Once it's dry, give it a coat of primer and finally a coat of the same gloss paint.