One of the frequent complaints women express about swimsuits (handmade and ready-to-wear) is that they don't provide enough bust support. There are two easy ways to add bust support to a swimsuit - add boning and/or cups. Even those of us who aren't full enough to need extra support can enjoy the benefits of boning and cups, as they also help the swimsuit stay in place and hold its shape.
Boning provides a little support and helps swimsuits (especially bikini tops) keep their shape. It can be inserted into almost any swimsuit when you are making it - all you need is a side seam. After sewing the side seam, sew another seam 1/4 inch from the side seam to create a vertical channel. Insert a piece of plastic bone (that you have cut to size and rounded off the end) into the channel. Keep in mind that your bone needs to be shorter than the length of the unfinished channel because you still need to sew the elastic at the top and bottom.
If you want to add boning to a one-piece suit, you will still need to make the boning the same length as the bikini (5 inches is a good choice). Sew the channel in the same way, but bring the bottom of the channel down about 5 inches and sew with a horizontal seam on the channel.
If you want to add seams to a ready-to-wear swimsuit, sew a swimsuit lining on the inside of the swimsuit at the side seam, insert the seam allowance, and then stitch at the top and bottom of the channel. Or, to sew a channel seam, cut a small slit in the lining at the top of the channel, insert the boning, and sew the seam closed.
Inserting the cups into the swimsuit when sewing the suit is as easy as sliding the purchased swimsuit cups into the suit fabric and lining when making the suit. Depending on the construction of the suit, the cups may hold themselves in place or you may need to secure them in place from the inside. Before securing them in place, try on the finished suit, place the cups in the position that best fits your individual body, secure them in place, then remove the suit and secure them.
To insert the cups into a ready-to-wear swimsuit, you can cut a small slit in the lining fabric, slide the cups in, and then secure them in the same way. You can close the seam or leave it open, as the lining fabric will not come apart.
If you can't find a swimsuit cup that fits you, or if you want the extra support of a steel bra, consider switching to an older (but well-fitting) bra. Cut your bra in half and remove the straps. Cut a piece of swimsuit lining, cut an X in the middle (the size of the X depends on your cup size), push the cups through, and secure the cups in place.
Sew the lining to the cups, being careful not to touch the steel ring, and use the lining piece in the normal swimsuit construction. You can machine sew the cups themselves or hand sew the edges of the cups.