By "knock down" I assume you mean remove the old popcorn stuff in order to spray on new?
I watched a professional spray LIGHTLY with water while scraping it off with a wide putty knife. Make sure you cover EVERYTHING with plastic first. It will make a mess.

No, we have built an addition with new drywall and want to do the "knockdown" technique on the ceiling. You spray a mix of joint compound & water and then use some kind of a tool to smooth down the popcorn spray. What tool do you use and also should we primer paint the new drywall before spraying the mixture and is 2-1 compound to water the right mix?

What you are referring to is a knock down texture(splatter and drag, monteray drag) Popcorn texture is accoustic that is sprayed on the ceiling and left as is, you cannot drag it.

To do this you splatter the mixed joint compound onto the surface evenly(experiment on how heavy you like it on a piece of scrap drywall), let it sit for about 10 minutes, then you drag it lightly with a 10 or 12" taping knife. Do not use a fan or moving air, this causes problems. Use a taping pan to keep your knife clean. As far as the mixture goes depends on how heavy you want it to look. Start off by dumping a box of mud into a 5 gal bucket, add water up to about 2 or 3 inches from the top, mix until smooth. You will need to make samples on a scrap piece of drywall, the machine you use has several size holes for different effects, if it is powered by an air compressor set it at about 40 lbs and go from there, if it is a texture machine (without a hopper) these are very fast and can cause a mess, be carefull and move swiftly letting go of the trigger promtly when stopping. It is ok to come down on the wall a bit, this is going to make a mess, expect that, you can remove the overspray with a taping knife easily when it is not quite dry (if it is new dry wall, if it existing walls with texture, I would suggest covering them with plastic). Cover all doors and frames, trim and windows. If it get on something, don't panic, this stuff (joint compound) comes off easily with a wet rag.

Prime the surface AFTER the texture is applied and dry. You can use a regular water based drywall primer. Then paint.