Transfer the measurement needed to the drywall sheet and mark it with a pencil. Place a T-square on the mark with the short arm resting on the top edge of the drywall. Using the T-square as a cutting guide, run a utility knife along the edge to score the drywall. Bend the drywall back until the sheet snaps. Cut through the back paper with a utility knife to separate the pieces.
For cutting openings for electrical outlets, use a key hole or drywall saw to plunge through the drywall.