I use a Porter Cable pancake compressor set at about 40 PSI, Sheetrock brand wall & ceiling texture dry mix, a Spraying Mantis spray gun. I mix the texture up using a mixing drill and paddle to a pan cake batter consistancy, making sure to mix the hell out of it. Any chunks will clog your gun and even worse small ones will show up all over the wall texture.
I tape everything off just like spraying a car. I shoot all the surfaces and go clean up the gun, compressor hose and anything else. Come back and check to see if the texture has set up a bit. When it's ready I use a wide plastic knock down knife and start in the first place I sprayed and work to the last place I sprayed.
I'm not the best at this so I often come back with a sanding pole after it all drys and go over it all really quickly and that usually makes it look great.
This is the knife I use for knock down.
A lot of other people probably like the squeegy looking knock down knife.
Keep in mind I only do remodeling so this is what works for me. Doing it on an entire house or on new construction would require doing it differently. I'm sure I have a lot to learn in regard to texturing and have a long way to go with it. It seems to be a real art to doing it well. I have hired professional texture finishers for jobs where texture matching was crucial.