check out some pictures of 20's dresses, find one you like and try to copy it. the whole 20's look was very stick-figured. women weren't supposed to have a waist or a bust. in the 20's art deco was in, so if you found a pattern or motif you like from that period you could somehow copy it onto the dress. oh and if the dress had any waist at all it was very low, it landed at the lower hip. you could add trim or a belt-thing and that will add to it. i hope i could help. if you want some pictures check out www.costumes.org
there's a section of research and a section for sources. hope i could help! good luck.