You can spray paint it which is the easiest and fastest and does not disrupt the popcorn, OR you can roll it on by hand. This is tough because the popcorn will fall of to an extent and you need a heavy knap roller cover so that you can get on all sides of the popcorn. You will find out that you will need to roll the area out from all four sides so that the total area is covered.
Tarp the area out good, and wear eye protection as paint and popcorn will come down.
I would recommend to go out and buy a Wagoner Professional spray painter for about $50 and then get a flex hose for the painter so that you do not have to turn the painter on its side. This will do an excellent job and will be much faster. You wil get excellent coverage.