Hold your tape measure from one end of the ceiling outward across the joists (perpendicular to them), and measure to the middle of the last joist that's within 8feet of the wall.
Mark that measurement onto a piece of drywall. Use your razor knife to score and snap the drywall to the right size, using a T-square to keep the blade straight.
Set up two ladders. With an assistant, lift the cut drywall into place, with the cut end facing the wall and the other end lined up on the middle of the furthest joist (the one from which you measured). Shoot drywall screws all along every joist behind the drywall, putting them about every 6 inches.
Install the next sheet of drywall at the end of the first one, with the edge of it on the remaining exposed half of the joist. The other end should land at the middle of another joist (the sheets of drywall are sized to space evenly on properly spaced joists).
Repeat until you have to cut a piece to fit against the far wall, then hang the next course, starting from the opposite wall as the first. This will ensure that the ends of the drywall don't line up from course to course. Continue until you've covered the entire ceiling.
Press mesh drywall tape over the seams between the boards. Use a drywall knife to spread joint compound over the tape, making the compound just wide enough to cover the tape and very smooth. Let it dry for about 6 hours.
Spread on a second layer of joint compound over the taped lines, making the compound wider. Let it dry and repeat a third time. Let it dry overnight, then smooth it out with a sander. The ceiling is now ready for painting.