Select a sheet that is large enough to cover your sleeper sofa. Choose a color and pattern that is appropriate for your room's design.
Place the sheet over the sofa, centering the sofa beneath the sheet.
Tuck the sheet around the seat cushions along the back edge of the seat.
Tuck the sheet between the seat cushions and the armrests.
Make a form-fitting cover by securing the edges of the sheet with safety pins, following the outline of the sleeper sofa. Do not do this if you intend to frequently use the sleeper sofa as a bed since the safety pins make the sheet more difficult to remove. Thread buttons or beads on the safety pins for a decorative effect. Make a more elaborate closure if you have the time and skills by sewing buttonholes and buttons on the sheets in lieu of safety pins. Add buttons only if you don't mind permanently altering the sheet.