Now that you know how to make a two piece swimsuit, it is very easy to add a little extra fabric and sew it together into a one piece.
Begin with your pattern for bikini bottoms. Measure your side from your bra band to just bellow your hip. Extend the pattern for the bottoms up until both sides are this length. For the back, draw a line straight across connecting these sides. In the front, connect the two sides with a peak in the middle.
Cut out the front, lining, and one strap like you would for a bikini top. You will not need a second strap. This swimsuit only ties at the neck, not at the back, so the single strap loops around and ties to itself (it’ll make sense when we get there).
Gather the center of the top front (like for the bikini top), pin this piece to its lining with the right sides facing in, and sew along the top edge. Turn right side out and, holding the main fabric and the lining together, sew the bottom edge of this piece to the top edge of the front bottom piece. The front of your swimsuit is now complete! Pin the front to the back with the right sides facing in, and sew up the sides and the crotch.
Following the instructions for bikini bottoms, put in the lining and add elastic to the leg holes.
Fold the straps in half with the right side facing in, and sew up each side, leaving enough space in the center for the sides and back. Sew the unfinished edge of the strap to the swimsuit along one side, across the back, and up the other side.
Take your new swimsuit to the beach!