Paint the ceiling with primer, applying it with a roller. Let it dry overnight.
Apply the textured paint to the ceiling with the roller. Apply it very thickly to one section at a time, about five feet by five feet.
Work the paint with your trowel, according to your preferences. You can press the trowel into the paint and pull it straight down, creating peaks and ridges; or press the trowel into the paint and swirl the paint into circular patterns; or set random lines in it with the long edge of the trowel.
Repeat the process for the next square section. Continue until you've done the whole ceiling.