Trowel the mud onto the ceiling manually to create a custom texture. Spread the mud in various directions to achieve a look and feel you are comfortable with. There is no wrong or right way to spread mud on the ceiling, so you can experiment with how much mud you apply until you are satisfied with the finish results.
Spray the mud onto the ceiling with a drywall sprayer for a popcorn texture. Load the drywall mud into the sprayer machine’s container and hold the hose firmly with both hands. Pull the trigger with the tip pointed toward the ceiling, and spray until you are satisfied with the amount of texture on the ceiling.
Let the popcorn texture dry for 20 minutes or so, and smooth it down slightly with a trowel to achieve a knockdown texture. Do not smooth the popcorn texture down completely. Instead, apply only light pressure so that you “knock down” the high points and end up with the classic knockdown look.