Remove the upholstered back and seat of the desk chair, using the screwdriver. Remove any of the old fabric. Use a flat-head screwdriver to remove any staples if necessary.
Place your foam on a flat work surface and top with the seat and/or the back of the desk chair. Trace around the seat, using a marker.
Cut the foam out to the right size and shape, using the electric knife. Smooth out any rough edges by trimming with the scissors.
Lay out the upholstery fabric on your work surface with the right side facing down. Center the foam and the desk chair seat on top.
Cut out the fabric, using the scissors. Cut around the seat, leaving 5 inches of excess fabric.
Pull up the fabric on one side of the desk seat and use the staple gun to hold the fabric in place. Go to the opposite side and do the same. Repeat for the two other sides.
Continue stapling around the seat until you staple down all the fabric. Pull the fabric tightly for a smooth seat.
Trim off any excess fabric. Replace the backing on the seat, if it has any. Replace the seat on the desk chair.
Repeat for the back of the desk chair if upholstery is necessary.