Before you start, have a section of the wall tested for asbestos contamination. If it is, you should hire a contractor to remove the popcorn ceiling. Working in an environment with asbestos in the air is dangerous and even more effort to work around.
Take everything out of the room, and hang plastic sheeting over the walls and floors. Cleanup will be extremely easy when you only have to roll up the plastic floor cover to collect the fallen popcorn debris.
Spray the popcorn ceiling in small sections with water to soften the texture. This will make it easier to scrape off as you go along so sections don't harden before you reach them. Don't worry about collecting the texture; just let it hit the floor as you scrape it off with a putty knife.