1 twin size flat sheet
about 30 small safety pins.
Start by folding the sheet in half with longest edges together. Measure and mark about 21" from hemmed edges (depending on the actual width of the sheet). Cut on your mark so you end up with 3 sections that are about 21" wide. Hem raw edges with a rolled hem.
Begin sizing the first section of fabric for the foot of the bed by folding first section in half (for twin size).
For a larger size bed, fold in ends so that fabric wraps around each corner about 6". You may want to make a pleat in the middle of the end if it is a wider bed than twin. Pin to box spring fabric covering across the end and both corners while keeping 1" from floor.
For sides of the bed, begin pinning second fabric section at the head of the bed. Place the fabric so it is 1" from floor and secure with safety pin at edge. For this standard height bed frame the fabric is folded over quite a bit, for a higher frame, there should be enough width in the fabric and would be folded over the edge less than this size.
Continue pinning about every 10" to center. Fold fabric into a double pleat and pin.
Continue pinning down the side from the pleat and fold the fabric under so that it meets the end of the bed and over laps the end section that is wrapped around the corner.
Secure corner with pins. Repeat on opposite side with 3rd section of fabric.
There you have a quick bed skirt for about $5 that can be reused on any size bed.