Scoop a generous amount of plaster to the hawk. Make sure the plaster is balanced on the center of the hawk. This will help lessen fatigue to your wrist as you work. Turning the hawk occasionally as you work will also help reduce fatigue over time.
Take the trowel and scrape off any excess plaster around the mound in the center of your hawk. By slicing the plaster, you create a seal around its outside edges, thus holding the plaster into place. (One of the main objectives of these steps is to keep gravity from causing the plaster to fall off the hawk as you work.)
Give the hawk a slight bounce as you hold it. That will lower the mound of plaster's center of gravity, and will again help hold the plaster in place. The hawk should now be facing toward your body.
Hold the trowel horizontally, so that it's almost perpendicular to the hawk, then skim a layer of plaster off the hawk. Your trowel should now be in a position to go straight to the wall, and apply the plaster. An experienced plasterer can do these steps in almost one fluid motion, saving energy and preventing arm fatigue over the course of a project.