Cut length of elastic for a snug fit.
Lower % = tighter. Waistbands are often 90\u201395% of the body measurement.
The cut length of elastic is the body measurement reduced by the snugness percentage, plus an overlap for joining the ends. For a 30 inch waist with 10 percent snugness and a one inch overlap, the elastic is cut at about 28 inches, since 30 times 0.9 is 27 and one inch is added for the overlap.
The right snugness depends on the elastic and the garment. Waistbands usually sit a little snug so they stay up, while cuffs and leg openings can be more relaxed, and wide, firm elastic is more comfortable than thin, tight elastic. Try the elastic around your body before you sew it in.
Braided elastic narrows when stretched, so it is better for casings, while woven and knit elastics stay flat and suit waistbands that are sewn directly to the fabric. Some elastics shrink when washed, so wash them first if the garment will be laundered.
Join the ends with a zigzag stitch or a few rows of straight stitching over the overlap, and stretch the elastic evenly as you sew it to the fabric, marking the quarter points to keep the gathers even.