A tongue & groove ceiling fits together under the same general principle as a tongue & groove floor, but the ceiling is actually a little easier to install, because it's softer, lighter wood and it doesn't require a special floor-nailer to secure the boards. A standard trim nailer is fine, though it isn't used in the same manner throughout the project. The first and last boards have to be top-nailed for solidity. For the rest of the ceiling, you'll be going in through the sides of the boards, so the nails will be hidden.
Locate each of the ceiling joists (the support beams behind the ceiling plaster) with your electronic stud finder. Mark the length of the location of the joists with your level and pencil.
Measure the length of the ceiling, in the opposite direction that the joists are running. Transfer that measurement to your first board. Cut it to size on your miter saw.
Hold the board on the ceiling, with the grooved side facing the wall. Shoot two nails into the face at each point where the board crosses a joist.
Measure and cut your next board. Press it in place next to the first one, locking the groove of the new board around the tongue of the one that was hung previously. Secure the board by shooting one nail in through the side of the board, just above the tongue, at an upward angle, at each point where the board crosses a joist.
Continue cutting and hanging planks across the ceiling, nailing them in from the sides.
Nail the last board up in the same manner as the first one (with two nails shot straight through the face at each point where it crosses a joist). Hang ceiling trim around the perimeter to hide the joint between the planks and the walls.