Mark the drywall sheet at the point where it needs to be cut -- for long cutting lines, pull the cord of a chalk line taut across the surface of the drywall sheet. Lift the cord an inch from the drywall surface and release it to cause it to spring back and produce a straight cutting chalk line.
Place the tip of a utility knife's blade against one end of the chalk line. Press slightly so the blade cuts through the drywall paper, and run the knife slowly along the length of the line. Bend the drywall sheet against one knee on the opposite side to the cut line -- this will break the sheet along the line. Run the tip of the utility knife along the length of the inside break, to completely separate the drywall sheet.
Run an old tube of lipstick or a large felt-tip pen several times around the outside front edge of all outlet/switch boxes, in the location where the drywall sheet will be installed.
Position the drywall sheet against the wall in the location where it will be installed. Push against the front side of the sheet in the areas where the boxes exist -- this will create the boxes' perimeter markings on the back of the drywall. Use a drywall saw to cut around the marked perimeters before installing the drywall in place.