Getting a bathing suit to fit properly can be a challenge. Often the bottoms are too tight or too saggy for the rest of the suit. You can only make a too-tight suit larger if it comes with strings or ties to adjust the size. You can make a suit smaller by adjusting the ties and sewing loose areas along the waistband or leg holes of the bottoms. Try using the tie method first, as some sewing modifications to a suit bottom may be visible.
Put on the bathing suit. Wear the suit as you normally would, without the addition of underwear. If you are modifying the suit, you will not be able to return it anyway.
Note any ill-fitting areas. It is nearly impossible to make the bottoms larger--unless the suit is starting to sag--but you can make bottoms smaller. Note any area where the bottoms sag or have gaps. Typically, this is around the legs, waist and along the seat.
Adjust any straps or strings that come with the bathing suit. Sometimes you can make the suit fit better just by adjusting these attachments. Make the ties looser to make the bottoms larger, or tighten the ties to make the suit smaller.
Check for any sagging once more. If you still have fit problems, gather the elastic seam along the legs or waist. Pin the band together with a safety pin. Remove the suit.
Fold the pinned areas together. Sew the pinned areas by hand with a needle and thread. You may need to use a thimble to get the needle to poke through the elastic. Sew through the pinned areas with two lines of stitches and secure with a knot.
Put on the bottoms and check the fit again. Make any additional adjustments for fit and sew in place. Repeat the process until the bottoms fit how you want them to.