Cover walls or ceilings you do not want texture on with plastic sheeting. Run painter's tape around the plastic sheet to attach it to the walls. Make sure the tape is pressed firmly to the walls or ceiling so it does not fall down during the texture application.
Prime the wall or ceiling with a lightly tinted wall primer. The tinted primer will make it easier to see if you missed a spot. Apply the primer with a roller so the entire surface is covered. Let the primer dry completely per the packaging instructions.
Mix powdered drywall compound into a 5-gallon bucket with water until the compound has the same consistency as a thin pancake mix. Stir the compound and water with a ribbon mixer attachment fitted to a drill. Allow the mixture to set overnight to dissolve lumps.
Stir the mixture with the ribbon mixer and drill. Create a strainer by setting a mesh screen on top of a 5-gallon bucket. Pour the mixture through the mesh screen to remove the remaining lumps.
Pour the mixture into the hopper gun by following the hopper gun's instructions. Connect the air compressor to the hopper gun, following the hopper gun's instructions.
Set the air pressure on the compressor to 25 to 45 PSI. Set up a scrap of drywall to practice on. Spray a widely spread, even layer of the compound mixture over the scrap. Practice until you feel comfortable using the hopper gun on your walls or ceiling.
Spray the compound over the walls and ceiling in a wide, evenly spread layer. Apply more coats over the first layer if you want more texture. Let the texture dry.