Plastering a large ceiling can seem like an overwhelming task. Completing sections of the ceiling can make the feat more manageable and feasible. Creating a grid on the ceiling with a chalk line will help guide you and allow you to more easily estimate the amount of time the project will take. If you are planning on finishing over a period of days or weeks, you will need to let each section dry completely before continuing. Start by moving furnishings and other objects out of the room and covering the floor in plastic sheeting.
Measure the ceiling for your grid pattern. Entryways and other small connecting areas can be marked off from the main room and done separately. This will keep your main grid as rectangular as possible. Decide how many sections there should be. Measure and mark these along the ceiling edges with a pencil.
Make the grid lines using your chalk line. Have a helper pull one end of the chalk line across the ceiling while you hold it at the opposing point. Hold the line taut. Pull the line away from the ceiling with your other hand, while your helper does the same thing on his end. Allow the line to snap back at both ends, creating a chalk mark across the ceiling. Repeat the process if you need a more solid and visible mark. Complete the entire grid, making the lines from the correct points you measured.
Place some of the plaster into your pan or hawk. Use your trowel to scoop out and spread the plaster, starting in a corner of the room and making sure to go the edges of your grid lines. Smooth out the exposed edge, lightly scraping it with your trowel, held at a slight angle. This will remove any large bumps or gaps between the plaster and ceiling and make the adjacent square easier to blend after this section has dried. You also can immediately move on to the next square and try to complete as much as possible in one session.
Lightly sand down the edges of the square you plastered after it has dried. Remove any remaining bumps or raised areas. When plastering the adjacent square, you will easily be able to blend in the new plaster over the smooth, rounded edge of the dried plaster square.
Continue the plastering process in the grid until the room is complete. When all the sections of the ceiling are dry, lightly sand out bumpy areas or patch holes you may have missed.
Always wear protective goggles and a respirator when applying and sanding building materials.