This is the inside of a string bikini bottom that I crocheted. The liner is important if you plan to wear one in public. Plus, if you plan to make and sell crocheted bikinis, you will sell a whole lot more with a liner in place. Since people will pay twice as much for one that is lined, you can see the value in learning how to put one in.
First you crochet the bikini. Then you take the fabric you want to line it with and cut it out about one inch larger than the crocheted piece. Using safety pins, pin the liner to the inside of the suit.
Hand sew the liner in, starting in the crotch area. Fold the fabric under about 1/2 inch as you stitch your way up and around to the waist. Stop. Go back to the crotch area and begin sewing the opposite side up to the waist. Continue in this manner until the entire leg opening has been stitched. Then sew the waist.