It's not asbestos?
If not, then the greenest and basically the only real way to do it is by:
- misting it with water and letting it soak in some.
- Then scraping it off with a drywall knife.
It is best to place some plastic down on the ground to capture the mess. You can then apply a skim coat, fix any issues & get on with your finishes.
For more information:
Read "How do I handle a "popcorn" ceiling that may contain asbestos?" a Q&A answered by Molly McCabe.