Reminder: You will need less than 1 yard swimsuit fabric, a very small piece of lining fabric (can be replaced with more of the main fabric or left out entirely), 3 yards 1/8 inch elastic, and matching thread.
Cut out the front and back from the swimsuit fabric and the liner from lining fabric.
Pin the front and back pieces together, with the right sides facing in. Sew up both sides and along the crotch, using a slight zigzag stitch. Pin the liner to the inside of the crotch. I used to sew it in place separately, but Prima who did this part on 5 swimsuits this weekend reports that there’s really no need – you can sew it in with the elastic.
Sew the elastic to the wrong side of the fabric at the edge of the leg hole, using an 1/8 inch zigzag and beginning at the crotch seam. Hold the elastic tight, but don’t stretch it. When you reach the end, cut off the extra elastic. Fold the edge of the leg hole over so the fabric wraps over the elastic. Fold again. Sew down this folded edge, stretching the fabric and elastic slightly as you sew.
The simplest way to finish the top is to put in elastic, just like you did for the leg holes. It’s easier to sew this seam, since it’s straighter.
Alternatively, you can sew on a waistband. To do this, cut a strip of fabric as long as the waist of your swimsuit and 3″ wide. Fold this piece in half lengthwise with the right sides facing in, and sew up the short end. Fold the loop you’ve just created so that the right side of the fabric shows on both sides. Sew the raw edge of the loop to the top of your swimsuit using a slight zigzag stitch.