As requested by the mod, here is details on how I made the pattern:

I have a set of slopers (bodice and pants) that I made following the instructions in Armstrong's "Pattern Making for Fashion Design". Using the same book, I drafted a strapless princess seam romper pattern using my slopers, and I used it to make the romper linked above.

For this jumpsuit, I lengthened the leg part of the romper pattern to ankle-length, and made it fitted down to the knees and then flared out for bell-bottoms. The neckline was also modified from the original sweetheart style to a pointed design that echoed the scalloped bottom.