Move as much of the furniture as possible out of the room that you will spray. Cover the floor with old bed sheets. Gather the furniture together that you are not able to move out of the room into a spot in the center of the room. Drape plastic sheeting over the furniture to completely cover it and protect it from the spray. Remove light fixtures or cover them with plastic. Wipe away cobwebs and accumulated dust from the ceiling surface using a duster.
Apply masking tape to the border between the ceiling and walls. Remove any drapes and tape plastic sheeting over windows. Protect the walls by hanging plastic sheeting or old sheets over the walls with masking tape.
Pour the appropriate amount of ceiling spray texture into a bucket. Add the proper amount of water, according to the manufacturer's instructions. Install a paint mixer bit into a power drill and stir the mixture, with the drill on low speed. Scrape the mixture from the sides of the bucket, using a paint stir stick, and stir the mixture again with the drill and paint mixer bit.
Put on protective equipment, which includes gloves, goggles, and a dust mask. Install the hopper to the top of the sprayer gun. Attach the air hose from the compressor onto the bottom of the sprayer gun. Pull the sprayer trigger to make sure it sprays out air properly. Hold the sprayer upright, along with the attached hopper, and add the spray texture mix into the hopper. Add an amount that is comfortable for you to work with, being sure not to over-fill the hopper.
Spray the ceiling, starting in a far corner. Grip the handle on the rear top of the hopper with one hand, while squeezing the sprayer trigger with the other. Adjust the sprayer's trigger adjustment, if necessary. Use a side-to-side, oscillating motion and work across the ceiling to the other end. Spray from end to end, working back and forth across the ceiling. Spray a thin layer, from a distance of 12 to 18 inches away from the ceiling surface. Stand on the steps of a ladder, if needed. Allow 24 hours for the texture to dry.
Spray a very light, second layer of texture on the ceiling, using the same side-to-side motions as in Step 5. Wait 15 minutes. Drag a knockdown knife gently across the ceiling. Apply a light, even pressure to scrape away any over-spray from the texture.