Measure the entire circumference of your box spring with a tape measure. Also measure from the floor to the top of the box spring.
Take the circumference measurement and double it. Use the doubled number to measure and cut the length of your fabric. Then measure and cut your fabric to the height measurement you took, plus two inches. So, for example, your dust ruffle would be about 20 inches deep by about 24 feet long, give or take a foot, if you were making it for a king-size bed.
Fold the top edge of the long side of the fabric down about an inch and a half. Use a sewing machine to place a seam about an inch down from the top edge of the long side of the fabric. This will create a space for the elastic to pass through. Make sure the top edge is folded toward the inside of the fabric, not the outside.
Fold the bottom edge up about a quarter of an inch. Again, make sure the fold is toward the inside of the fabric. Select a tight stitch on your sewing machine, then sew a straight line at the top of the folded edge to create the bottom seam.
Measure and cut the elastic to the size of the circumference of the box spring minus about 4 inches. Affix a large safety pin to the end of the elastic. Push and feed the elastic all the way through the casing you sewed at the top of the dust ruffle.
Sew each end of elastic to the edge of the dust ruffle and create a seam on the ends of the fabric. Do not sew the ends of the dust ruffle to each other. Attach some hook-and-loop fasteners at regular intervals to the box spring and to the dust ruffle. Take care to spread out the gathers along the elastic strip in an eye-pleasing fashion as you attach the dust ruffle to the box spring.