Add water to the drywall mud. Use 4 ounces per gallon of mud. As the volume of water is small, you can pour it directly into the drywall mud container.
Put the mixing bit into the drill and tighten the chock. Submerge the end of the mixing bit into the drywall mud and start the drill at a slow speed. Increase the speed gradually and move the bit about the container until the mixture is uniform.
Pour out some mud into the well of the tray.
Put the crow's foot roller on the roller sleeve.
Roll the roller into the tray. Roll off any excess on the ramp. You want a full roller, but not a dripping roller.
Roll the texture on the wall. Use slow strokes. This prevents spattering. For regular patterns, work in vertical strokes. For irregular patterns, mix up the stroke direction.
Work in sections of about 5 square feet at a time.
Allow the wall to partially dry -- around 15 to 30 minutes. Check the height of the texture. If there are areas that are too high for your taste, flatten them with smooth swipes of the knockdown knife, sweeping across the partially dried mud at the maximum height you want for the pattern.