Place the box spring and mattress on top of the metal frame.
Drape the fabric over the bed, wrong side out, arranging it so that it falls evenly on all four sides of the bed. Pull the fabric together at each corner to form a vertical straight line down the side from the top corner of the mattress to the bottom corner of the box spring on all four sides. Pin the fabric together along the line at each corner from top to bottom. You are not tucking the fabric underneath the mattress as you would a normal bed. You want the fabric to conceal both the box spring and mattress and form a box around the bed.
Fold the bottom edge of the fabric up 1 inch on all four sides and pin in place.
Take the fabric off the bed and sew along the pinned lines, removing the pins as you go. Also sew along the bottom edge of all four sides to hem the fabric cover.
Trim excess fabric from the corners and turn the fabric right side out.
Slide the fabric over the mattress and box spring again. Smooth it out so that it is even and flat.
Pile the cushions along the back of the couch. Place one cushion on each side for arm rests if you would like.