Drape the sheet over the chair. Be sure to use a sheet that reaches the floor. The point of the chair cover is to hide what it looks like underneath. Using a sheet with ruffled edges is an easy way to add detail to the cover without additional cost.
Pinch one side, and safety pin it from top to bottom down the side. Repeat this for the other side as well. This will prevent the chair cover from moving around the chair when you sit in it, as well as completing the cover.
Place ribbon around the base of the chair where the backing meets the seat. Add an additional 1 to 1 1/2 feet to the ribbon before you cut it.
Wrap the ribbon around that base and tie a large bow in the back. Gather the material along the bottom of the chair, and pull it all to the back.
Tie a knot with the remaining material in the back of the chair. Pull the sheet as tightly as possible to create a very fitted chair cover.
Secure the knot by wrapping the rubber band around it several times until you can twist the rubber band no longer. The tighter the band is around the sheet, the more likely it is to be held in place tightly around the chair.