Instructions
Buy polyester thread for the needle and polyester or texturized nylon thread for the bobbin. These threads are essential if you want your alterations to last in pools as well as at the beach because they don't break down quickly in chlorine.
Measure your swimsuit alterations a few times to make sure you cut out the right amount. Before you actually cut it, pin the swimsuit and try it on again. You can always cut off more fabric or take in a larger amount, but it's very hard to add fabric into a swimsuit.
Use a ball point or stretch needle to sew swimsuit fabric in order to keep the fabric threads from ripping. Practice sewing on a piece of test fabric to make sure you don't bunch the fabric or rip the fibers as you sew.
Alter your swimsuit by cutting the lining open at the seams. You may be able to just rip the seams out, but be careful not to rip the fabric or tear the fibers of your lining. If your seams are attached to the swimsuit, simply cut the lining as close to the seam as you can.
Sew your swimsuit alterations without cutting any pieces off. After you take in the parts that don't fit, try on the swimwear. Once you ensure the proper fit, you can cut the parts off and serge or fold over the edges, then sew the lining and swimsuit together.