I did it last night as a matter of fact. It's really messy so make sure you put a drop cloth down. There are 2 places that I found with info on this www.homaxproducts.com and do a search for Ron Hazelton. I also found a couple of message boards and I think they may have been at this sight.
Anyway, if you have painted over the popcorn, forget it. We did a small spot last time we painted as an experiment and it was murder to get the popcorn off. Maybe if you sanded it so that it would soak up water it could be done more easily. The key was water. The more you can put on the easier it comes off. I used a spray bottle and my fingers were killing me. Next time, I would use a deck sprayer or weed sprayer as suggested by others. The other thing (cont'd)