Find each joist in the bathroom ceiling with an electronic stud finder. Mark the positions of the joists with a level and pencil.
Measure the edge of the ceiling where you’re going to start. The starting wall must be perpendicular to the joists.
Cut a ceiling board to the length of the ceiling, as you measured it perpendicular to the joists. Make the cut on your miter saw, straight across.
Set the board to the ceiling, at the starting edge, with the groove of the board facing the wall and the tongue side facing out into the room. Leave a half-inch space between the board and the wall.
Secure the board with nails shot straight up through the board’s surface at each point where it crosses a joist. Shoot in two nails per joist.
Cut a second board to fit along the length of the ceiling, which will be the same length as the first board. Hold the second board with the grooved side over tongue of the first board. Affix the second board with your trim nails, shooting the nails through the sides, one at each joist.
Repeat for each ceiling board, building across the ceiling. Cut the last board lengthwise to fit against the far wall, leaving a half-inch gap there. Ceiling trim will cover the gaps.