Mix the joint compound by scooping 2 gallons of compound out of the bucket, into a clean 5-gallon bucket.
Add 4 oz. water to the bucket.
Attach a cast head mixer to a drill and slowly stir the contents. Hold the bucket securely between your feet as you mix, or have someone assist you. The compound should look like smooth cake frosting.
Scoop up some of the compound onto a 1-foot trowel and spread it onto the ceiling so it's 1/8-inch thick. Start at the edges and work into the center until the entire ceiling is covered.
Set your sponge in one corner on the ceiling. Hold one end still and spin the sponge around the end to create a circle. Create a row of circles across the length of the ceiling.
Start the next row of circles so that they overlap the previous row, turning the first row into half-circles. Continue in this way until the entire ceiling is covered.