Measure for your outlet boxes and transfer the measurement to the Sheetrock if you are going to measure everything. Ensure you make straight lines by using a carpenter's square. Alternatively, coat the edge of the outlet box with blue chalk or lipstick. Place the sheetrock in place and press in around the outlet area. Lift the Sheetrock straight off and the chalk or lipstick will leave an outline on the Sheetrock.
Press the end of your drywall saw into the Sheetrock and cut out three sides of the outlet. Hold the saw at a slight angle to cut the inside portion of the hole slightly larger than the face. This helps when installing the Sheetrock over the outlet.
Score the fourth side of the outline with a utility knife. Cut all the way through the Sheetrock until you reach the paper backing on the opposite side.
Push the outlet piece toward the side with the backing still attached. Cut the backing at the crease with the utility knife.