Plasterboard is the modern replacement for plaster, forming walls and ceilings by fixing broad sheets of gypsum along the wall studs and ceiling joists. The system is a great improvement over the old plaster and lath process, but there is one big drawback: if the joists in the ceiling are not at exactly the same level, then the plasterboard itself will be bowed. Ceiling joists should all be all the same level along the bottom, but they can move over time. If this has happened to yours, you should not drywall until you have added new joists to the sides of the old joists in order to level out that surface for the plasterboard.
Hammer 9 cm (3 1/2 inch) nails along the tops of the two walls that run parallel with the ceiling joists, putting the nails every 90 cm (3 feet) along the wall in a straight line 7.5 cm (3 inches) down from the top of the wall. Use your level to make sure the nails are in a level line and are at the same level throughout.
Stretch a string tightly between each pair of nails on opposite ends of the room, so the string runs perpendicular to the joists and a few inches under them. Make sure the strings are all level.
Affix aluminium or wood joists alongside each of the existing joists, with the new joists hanging lower than the original ones. Make the bottom edge of each of the new joists even with the strings all the way across. Secure the new joists to the old by shooting 7.2 cm (2 1/2 inch) wood screws through the sides of the new ones and into the old ones. Put screws along the upper and lower edges of the joists, every 60 cm (2 feet).
Measure the length of the ceiling, perpendicular to the joists, from one wall out to the last joist within 2.4 m (8 feet). Take the measurement from the middle the new joist edge.
Cut a piece of plasterboard to that length, scoring the plasterboard along the edge of your level with a utility knife. Snap the piece at the scored line.
Set the plasterboard up to the ceiling, with the cut side toward the wall and the factory edge along the middle of the new joist. Secure it with plasterboard screws every 15 cm (6 inches) along each joist.
Hang the rest of the first course of plasterboard end to end along the entire length of the ceiling. Hang additional courses over the rest of the ceiling, arranged so there are no four-way intersections between the plasterboard sheets.
Press plasterboard tape over the seams of the plasterboard. Spread joint compound over the tape with a plasterboard knife, in a smooth, flat layer. Let it set 6 to 8 hours.
Apply two more layers of joint compound over the seams in the same manner, keeping each layer flat and smooth and letting each one dry completely. Sand the final layer with a drywall sander to get it completely smooth. The ceiling is now ready for painting.