Tape the top of the wall around the perimeter of the ceiling with painter's tape and attach a length of 12-inch masking paper. Cover the floors and all woodwork with plastic sheeting, drop cloths or masking paper.
Mix 2 gallons of all-purpose drywall joint compound with water in a 5-gallon bucket until the mud is pourable. It should be the consistency of a thick milkshake -- the thinner it is, the finer the texture it produces.
Put a 3/4-inch-nap roller skin on a paint roller and screw the roller onto an extension pole. Dip the roller in the texture and roll the ceiling as if you were painting it. Roll completely in one direction, then roll again in the perpendicular direction to even out the layer of mud.
Screw a slap brush onto the extension pole. Several types are available at any hardware store, but the most common is oval and resembles a large hairbrush. You may find two of these joined by a wooden connector.
Roll a little mud onto the brush with the roller to prime it, then slap the brush against the ceiling to produce the texture. Use moderate force. If you do it too lightly, the brush accumulates mud, but doing it with too much force may loosen drywall fasteners.
Rotate the brush through a half turn before each slap to randomize the pattern, and overlap the pattern by about a quarter of the brush width. Rotate before the brush contacts the ceiling, not while it's in contact.
Let the mud dry overnight after you've finished stomping. Roll the ceiling with drywall primer before you paint it.