Prepare the ceiling by taping out all joints and removing light fixtures. Cover a finished floor with a non-slip paper drop cloth.
Thin premixed drywall compound with water until it is the consistency of a thick shake. It should drip from a spoon, but not be runny.
Fit the drywall spray gun with a pump hose and put the end of the hose in the bucket of thinned drywall compound.
Spray the ceiling in thick, but not runny, sections of texture, overlapping each strip slightly. Spray slowly and evenly, covering the entire ceiling so no bare spots remain.
Wipe only the tips of the texture with the wide edge of a drywall trowel or squeegee. This creates the knockdown look. The tips of the texture are pushed down into the texture. Operate the spray gun while another person follows behind and wipes the texture.
Adapt your strokes to create the type of knockdown you like the best. Pull a wide trowel from side to side for a basic look. Alternately, a shorter blade can be used in a half-circle motion to create a pattern within the texture.
Keep a wet edge all the time. This means you must not get too far ahead of the person wiping the texture.
Continue until you've applied the knockdown texture to the entire ceiling.