Measure the edge of the piece of drywall that must attach to the rafter, using a tape measure.
Mark a piece of 2-by-4 with a pencil to that length.
Cut the 2-by-4 using a circular saw on the pencil mark you made. Wear safety goggles when cutting the wood and keep your hands away from the blade at all times.
Attach the 2-by-4 to the rafter using eight-penny nails and a hammer. Ensure that the bottom of the new piece of wood is flush with the bottom of the rafter. This process is called sistering a rafter.
Secure the piece of drywall to the sistered rafter using an electric drill with a driver bit installed and drywall screws.