Tape off the walls at the point where they meet the ceiling. The top of the tape should end at the top of the wall. This will keep plaster off your walls, and speed up cleaning time.
Make a batch of skim coat plaster. Make sure mix is smooth and even.
Starting at a corner of the room, scoop up some of the plaster with the drywall trowel. Apply to the ceiling, pushing it on with the drywall trowel. Cover only about a 3x3-foot area, but leave the edges random. You don't want your start-stop areas to be noticeable. Apply a fairly even coat of about 5mm.
Scoop up more plaster, and apply it over the fresh plaster, working it into a pattern or design. It is easiest to use a random pattern. The thicker you apply the plaster, the longer it will take for it to set. That gives you a little more time to go back and work your design. Refill your hawk with more plaster as needed.
Continue in sections until the ceiling is done. Using fresh plaster, go back over any areas that look uneven. Let the plaster dry for a couple of hours before removing the tape.