- 1). Begin by taking your own body measurements, marking your own sizes onto the pattern. Cut out the correct size. For example, if you're a size 10, then cut on the L (large). If you're an odd size, use an old swimsuit as a basic pattern, adding inches to the pattern as needed. Remember that the pattern is only a suggestion. A good rule of thumb is to use your body measurements plus 1 inch more, since you can always use that with seam allowances.
- 2). Cut the lining the same size. Make sure that the fabric is on the correct side facing down. Pin the lining to the fabric and sew all the way around, using the double needle for strength. Do this for both pieces of fabric. All of the body fabric should be lined, except the material for the straps.
- 3). Baste the suit together, lining side up. Start from the crotch, moving up the sides. The back and the top piece will have straps added, so just worry about the crotch and sides for now. If there is a concern about the fitting size, you can sew it with a very simple stitch so that you don't jab yourself with the pins, and then take in or rebaste the fabric as needed.
- 4). Sew using a zigzag stitch. The stitch must be thick given the two pieces of swimming suit lining and the thickness of the lycra. Refer to the pattern for seam suggestions, but it's usually between 1/4 or 1/6 of an inch in, depending on the size.
- 5). Cut the elastic and strap pieces according to the pattern instructions. Remember, if you added inches to the swimsuit size, then you will likely need to add inches to the strap size.
- 6). Pin in the elastic to the leg holes by folding the fabric over the elastic and stitching on the right side. The elastic will be slightly smaller than the fabric, so stretch it slightly. Continue to use the strong zigzag stitch with the double needle.
- 7). Fold over the fabric for the straps. They will be sewn shut on all three sides so that they're basically a long, thin piece of fabric. Baste them to the back of the suit and sew accordingly. The top strap can be connected or tied, depending on the amount of bust support needed.
- 8). Remove all the pins and cut off any excess strings. If you wish, you can add a bra cup to the front. Simply refer to the pattern instructions for additional inserts.
Source...