Eric, at Modern Self, found a great post called "How to transform ugly popcorn ceilings into something worth looking at within just a couple of hours." Pretty straightforward.

Evidently, Eric has been dying to make changes to his own popcorn ceilings, but as a renter his hands are tied when it comes to home improvement projects. When he does get his own condo he is going to rip it apart and with all his decor knowledge I’m sure it will be a fabulous project. Until then he will just have to put up with those ugly popcorn ceilings. The neat thing is, you don’t have too and here’s what you can do…..

By covering the existing popcorn ceiling with Polystyrene ceiling tiles, you will have a brand new look in
about two hours. The tools used for this project were: utility knife, putty knife for adhesive, miter box to cut the crown moldings, measuring tape and pencil.  Very simple. You can leave the tiles white or paint them.  Beautiful!