Attach a piece of sheet rock extending from the ceiling to 2-feet below the door's header frame by screwing the panel into the door's side studs at approximately 12-inch intervals.The bottom of the door opening will be left open to reveal the framing studs beneath.
Leave the bottom of the door opening uncovered to reveal the framing studs. Do not attach the sheet rock to the door heading until the opening is completely cut out.
Measure 1/2-inch inward from the inside edges of the side frames. Mark the points with your pencil.
Place your drywall square against the pencil mark so that it lies parallel with the side frame. Draw a line upward to just below the header. Mark both sides of the opening the same way.
Place your drywall square on the marked lines and score the sheetrock along the straight edge using your utility knife.
Place your drywall saw through the door opening below your sheetrock panel. Saw upward along the scored line until you feel your saw hitting the header.
Cut both sides of the panel using the method outlined in Step 6.
Score the sheetrock on the back side using your drywall square and framed header as guides. Apply pressure to the cut, causing the sheetrock to break cleanly.
Score the paper on the front side of the sheetrock. Remove the piece that is now completely cut.