Although swimsuits don't use very much fabric, they're one of the more challenging clothing items to sew, since they require stretchy fabric and a very close fit. However, sewing your own swimsuit will let you choose just the size and style you want to suit your body type. Many swimsuit patterns have multiple style options, such as a choice of one or two piece suits, different necklines or different hip heights. Some patterns also offer swimsuit accessories, such as cover-ups or skirts. Most fabric stores have a fairly limited selection of swimsuit patterns and fabric, so you if you can't find what you need locally, you may want to shop online.
Measure the fullest part of your bust, waist and hips using a soft, flexible measuring tape. (If you don't have a sewing measuring tape, you can cut a non-stretchy piece of ribbon or string to the right size, then measure it with a regular measuring tape or ruler.)
Choose a swimsuit pattern you like that comes in the size you need, following the guidelines on the pattern size chart.
Buy fabric for the swimsuit, following the guidelines on the pattern envelope for how much fabric you need and what type to use. Most swimsuits require slick, stretchy fabric made especially for swimsuits or active wear. Many fabric stores have an aisle of swimsuit or dance fabrics.
Buy fabric for the lining of the swimsuit, if needed. Many swimsuit patterns use the same fabric for the outside and the lining.
Buy matching thread, elastic and any other notions listed on the pattern envelope that you need to complete the swimsuit. You may also need or want to buy bra cups for the swimsuit, which are available in most fabric stores.
Cut out the pattern pieces for the swimsuit, using the cutting line for the size you need. Be sure to cut precisely so the swimsuit will be just the right size. If the swimsuit has several different style options, cut only the pieces you need.
Smooth out the patterns and press them with a cool (low-temperature) iron to get out the all the wrinkles. This will make the patterns more accurate, which is very important for close-fitting swimsuits.
Follow the cutting guide in the pattern directions to pin the patterns to the fabric and cut out the pieces.
Follow the directions that came with the pattern to sew the pieces of the swimsuit together. Make sure to use the exact seam allowance given in the directions, so that the swimsuit will come out the right size.