Hello there!
She’s an absolute classic, but also a doozy, considering her cartoon proportions. This is not a beginner project, so I’m going to assume you already have some dressmaking knowledge and don’t need the super basics like what type of pattern to use (find a strapless gown pattern, preferably with princess seams. You can hide the seams with careful cutting of the sequin material.)
My first piece of advice: saw “screw accuracy” and don’t have the low back. It would be nearly impossible to have the amount of bust support you will need while also keeping the low back.
My second piece of advice is to build a corset either into or under the costume. This way, you can get that bust support and maximum cleavage while also making your waist look as tiny as possible (even if you don’t tightlace past an inch or two, it will still create the illusion of a much smaller waist!).
My third piece of advice is hip padding to really push those proportions. Wear two or three pairs of dance tights over the hip padding to squish down all the edges and give a seamless look.
For the actual sewing, you will need structure, structure, structure. I would advise looking into how actual ballgowns and red carpet dresses are made – basically, you will use an understructure of much thicker and more stable fabric, bone that, possibly add some fusible fleece on top to smooth things, out, and then add the fashion fabric, which is the classic red sequin fabric. Even with a corset understructure, you will need this much support to keep the heavy fabric in place and smooth on your body. If you opt to not have a corset, this becomes even more important.
If you opt to go for the super low back…that is some highly advanced dressmaking, but I would recommend a Misses Kisses bra in that case if you want the extreme cleavage.
I hope that helps! The main problem with her dress is the proportions, not the actual sewing.
—Fabrickind / Q&A Staff / Twitter