Gypsum board for ceilings comes in two thicknesses: 3/sin and 1/2in, the former being suitable for use where the joist spacing is no more than l8in and the latter where the joists are up to 2ft apart. The standard sheet sizes are 8 and 10 x 4ft. You may find the smaller sheets easier to handle and you can cut them in half to make them even more manageable. The edges should meet on the joist centerlines, so you will probably have to trim them slightly anyway.
The first job is to nail lengths of 2in sq or 2 x Sin wood along the walls parallel with the joists so that its lower edge is level with the undersides of the joists. Then fit more short lengths of wood to the walls between the ends of the joists to provide support for the edges of the boards.
The sheets of gypsum board must be fitted with their long edges at right-angles to the joists. Toe-nail more lengths of batten to act as bracing between the joists so that the inner edges of the sheets will fall on their center lines. A length of batten marked with the board width will help position them accurately.
Finally, mark the position of each joist on the walls as a guide for nailing the sheets in place.
To cut sheets to size, use a utility knife and steel straightedge. Cut down through one face of the board, snap back the waste against a batten and run the knife blade down the crease from the other side.
If you intend plastering the ceiling, fit the gypsum- board gray side down. For painting or papering directly over the top, leave the ivory side showing.
Holding large sheets of board against the ceiling for nailing can be difficult so nail lengths of 2 x lin batten together to foiiu T-shaped props with which a helper can support it while being nailed in place.
Nail the first board in place, working from the center outwards and spacing the nails at 6in intervals. Drive them home so that they just dimple the surface; to be filled later. Use 11/4in gypsum board nails for thinner sheets and 11/2in for thicker kinds.
Continue in this way, working across the ceiling. Keep any cut edges up against the wall, but if this is not possible make sure they meet on a joist with a slight gap in between for filling; stagger the joints.
When you have clad the entire ceiling, seal the joints between the sheets and, if you prefer, apply a thin skim coat of plaster.