Fill the bottom third of a clean bucket with powdered or premixed drywall taping compound. Premixed is better for texturing.
Add water to the compound 1 cup at a time and use a heavy-duty drill fitted with a paddle attachment to blend the mixture until it's the consistency of thick cream.
Place a large paint strainer screen over the top of another clean bucket and strain the thinned compound from the mixing bucket into the clean bucket. Even small lumps can clog the spray gun so straining is essential.
Fill the reservoir of the spray rig with the strained mixture. If you’re using a hopper spray gun, the gun will have a large funnel sitting on top of the gun. Don’t fill the hopper more than a third full because the compound makes the gun heavy, and your arm will quickly tire.
Turn on the spray rig and test the spray on a scrap piece of drywall or plaster, adjusting the nozzle tip of the gun until you like the splatter pattern.
Start spraying the wall in an upper corner, moving your arm in wide, slow strokes to deliver a consistent texture pattern. Work back and forth in a 5-foot section, from the top of the wall down, then start on the next section immediately.
Continue until you’ve covered the entire wall with an even texture coat.