Measure your futon mattress and purchase sheets that match these dimensions. The average futon mattress will need two full-size flat sheets to cover it.
Wash and iron the sheets. Lay them out, right sides in and pin them along three sides. Sew the sides, leaving a 5/8" seam allowance.
Cut your ribbon into eight 7" pieces. Fold over both sides of each ribbon 1/8" and sew to prevent fraying.
Sew the ribbons on open edge of your slipcover to create ties. Make four marks on the open edge of your slipcover so the ties will be evenly distributed. Sew the ties so one is on the top and one is on the bottom, attaching about 1" of the tie to your slipcover to ensure stability.
Put the slipcover over your futon and tie it shut.