For "regular" popcorn, it is a pain--in the back, the shoulders, the arms--to remove, but doable, if you have the time and the will to do it. You can buy accessories to make it easier, but the basic way is to put plastic all over the entire room, floor, walls and especially openings into other rooms.

Buy a heavy duty air filtering mask and wear it, along with a hat or scarf and glasses or goggles or both together. Whatever clothes you wear, be ok with never wearing them outside the house again.

Get a clean spray bottle, and fill it with warm water. Spray the popcorn till it softens. Take a wide blade putty knife at a 45 degree angle, and scrape off the popcorn.

Keep doing this till the popcorn is gone or you are exhausted. Come back and do some more. Once the dust settles, find the areas that didn't get all the popcorn removed, and repeat.

When the popcorn is gone, find every place where the taping and mudding were less than stellar, and spackle and sand them so they're smooth. The reason that popcorn was sold to homeowners in the first place is that it covers a multitude of sins in the taping and mudding of ceilings.

Once the ceiling is smooth--also smooth out the inevitable gouges you will have made with the putty knife--you can put two coats of sealing primer on it. There is a special one for sealing bare drywall, which is what you now have.

Then go over it again with the flat ceiling paint. It comes in white, and can be used as is, or tinted, if you want a more subtle intersection with the walls.

Whatever you do, don't go shiny. There is no such thing as a perfect ceiling, and shiny will play up any small defects, as well as making the room visually shorter.

I did this in my dining room. If I do any larger of a room, I will hire it out. It was hours upon hours of filthy work. It's lovely now, but it was worse than having a baby--and I've had four, so I know! LOL.