Calculate the amount of fabric you would need for the sides if the sides and end were added as a straight piece of fabric. Double that amount to allow for gathering. Triple the amount if the fabric is very light weight or you want extreme gathering.
Attach the "ruffle" by sewing them to the top with the right sides together. Piping attached to the edge of the top of the bedspread help keep a distinct divider between the top and gather.
An alternative way to attach the gather is to hem both long edges of the ruffle and lay the wrong side of ruffle on the right side of the top, sewing just below the hem on the top of the ruffle, sewing it to the edge of the top with top stitching. A double needle or two rows of stitching adds a nice touch.