|I can't take credit for this tip - it's from Threads. It's a great technique for adding a full lining to the front of a swimsuit. Generally only the front of a swimsuit is fully lined.
Layer the pieces as so:
RS swimsuit front to RS swimsuit back
Place the RS of the front lining onto the WS of the swimsuit back
Stitch up the side seams and turn; the lining will match up to the front, and the seams will be turned to the inside.
The best way to hold the three layers together is to use a glue stick.
This works for both two-piece and one-piece swimsuits.
I'll post a photo this weekend, since I'm making swimsuits for my kids right now.