Drape the fabric over the entire sofa until it is covered completely, with at least an extra 6 inches of fabric hanging onto the floor on all sides. Tuck the sheet fabric firmly into each crevice between the cushions, the back, and arm rests. Begin with the area at the rear of the sofa cushions. Feel around in the crack for the wooden supports on which the sofa's weight is distributed. Staple the fabric to whatever supports are available at the back of the cushions, and continue to staple along the back of the cushions, stretching the fabric tightly between staples.
Stretch the fabric tightly over the top of the seat cushions, and find the similar wooden supports within the crack at the front of the cushions. Staple the fabric to these supports. Continue to stretch the fabric tightly while you staple along the front of the sofa in this crack. Skip this step if there is no crack and the staples would be visible, in which case, the staples at the sides and bottom of the sofa will have to be sufficient.
Staple the fabric on either side of the sofa cushions, including the crack that may exist between the sofa cushions. Follow a similar procedure to attach the fabric deep within any cracks in the arm rests or back rests, ensuring that the fabric stays tight throughout. Stretch the fabric tightly over the arm rests, and staple it high up under the outside of the arm rest, as far out of sight as possible.
Stretch the fabric over the back of the sofa. Flip the sofa upside down. Pull the fabric tight on the underside of the sofa, and staple it to the wooden supports under the sofa. Continue to pull the fabric tight from all sides, and staple it in place. Cut off any fabric that overlaps the bottom of the sofa by more than 6 inches, to minimize the disorder under the sofa.