How do I spray a knock down ceiling and what materials do I use

Hi first you have to have a ceiling that is pretty smooth if it was just drywalled or plastered you should prime it first to equalize the surface porosity then you need a hopper gun with a compressor you then spray the ceiling with drywall compound that is thinned down or you can use a drywall spray that you mix with water after the ceiling is sprayed let it set for a few minutes and then knock it down with a broad knife let it dry and paint it and it is done