Once the surface is covered, go back over it, smoothing the plaster to an even thickness. Do not try to achieve perfect smoothness yet (Image 1).
Leave the plaster to dry for another half hour, until it is harder, but still slightly damp (Image 2).
Sweep a clean, dampened trowel blade across the entire surface, smoothing the plaster and redistributing any excess to fill small indents. Hold the blade at a slight angle with only one edge on the plaster to achieve a smooth finish (Image 3).
Leave the plaster to dry for at least half an hour, until the surface is firm enough to touch without moving the plaster, but is still damp. Repeat the smoothing process, again using any excess surface plaster to fill small depressions. If necessary, use a wet brush or garden spray gun to dampen the plaster as you work. Aim for a smooth finish at this final stage; it is more effective than trying to sand rough plaster when dry (Image 4).
Use a small, damp brush to finish edges and corners neatly (Image 5).