Find each joist in the ceiling using an electronic stud finder. Mark out the positions of the joists across the whole ceiling, with a level and pencil.
Measure the ceiling length along the edge where you're going to start, on either of the two sides that are perpendicular to the ceiling joists. Mark out the measurement on a piece of the ceiling board. Cut the board on your miter saw.
Hold the board to the ceiling with the grooved side facing the wall and sitting out ½ inch out from it. Shoot trim nails up through the board's face, driving two nails at each point where it crosses a joist (according to your marks).
Cut the next board to size. Set the grooved edge of the board over the tongue edge of the first board, getting them snug and tight. Nail the second board through the sides rather than the face, driving the nails up through the edges at an inward angle and using one nail for each joist.
Repeat the process for each board, building across the ceiling. Use a table saw to cut the last board along its length so it will fit along the last wall with a ½-inch gap left there. Cover the gaps when you install the ceiling trim.