To do the back, you're going to go into the first chain you made at the very beginning (the edge of the crotch). Using this as a starting point, you will create the back the same way you made the front. You'll want to increase the stitches for the back more so than you did the front.

1. Double crochet the first row into the crotch
2. Start increasing into the first and last stitch for the next three or four rows
3. Start two increases from here on out (going into the first, second, second-to-last, and last stitches)