If you have popcorn ceilings then you need to scrape it off w/ 6 inch knife (drywall taping knife, not out of your kitchen -OK)
Then fill holes you made.
(if you have painted the ceilings and you have big dipples, you need to put a layer of concrete fill 20% w/ topping mud (I use synko clasic) with regular mud to fill these to flat).
Then 2 coats of classic drywall mud, sand between coats 100 grit and then 240 for finishing sanding..
Use a 3M Extra fine/fine sponge and a clear 150-300 w. clear light bulb to sand out imperfections..
my 5 cents worth.
Glenn S. Berlow, Author