If you really want to check the suit's shape or make major changes before sewing, I suggest making a test suit, as described in Drape a swimsuit that fits . Tips for Successful Sewing When cutting, lay out pattern pieces with the fabric's greatest stretch going around the body. Set up your sewing machine to get a perfect tension and stitch for the maximum amount of stretch in your seams. If using a serger, set the differential feed to the highest number. Tie off ends to prevent raveling.
Second row adds security.
Some people sew the seam once, stretch it until the thread breaks, then resew the seam using a double row of stitching. My favorite stretch stitches include some reverse stitches within the stitch pattern, which produces a very strong stitch that won't break. Better swimsuits have a full front lining, and it makes sense to construct yours to hide all the inner seams. Cut the front lining from the front pattern, stopping halfway up the shoulder straps to reduce bulk, and add bra cups or bandeau elastic, as explained below. When you're ready to insert the lining, place the front and back right sides together and the right side of the lining next to the wrong side of the back, then stitch the crotch and side seams.