I am going to admit right now that I am a lazy and impatient sewer, and avoid making the built-in bodysuit whenever possible. Depending on the neckline and backline of the dress and depending on the dress material, I often don't bother building in that bodysuit that you see in dresses. First off, it is easier to sew! And secondly, it's easier to sell the dress because the potential buyer doesn't have to worry about torso length or hip width as much. I have a long-line strapless underwire bra that extends down to my hipline, and it has a low back. This gives me a very smooth line and good support without having to fuss with inserting a bodysuit or bra cups into the gown. If the fabric you are making the dress out of is opaque enough and drapes well enough over your body, you can get away with just wearing colored dance trunks over your pantyhose. My favorite place for getting dance trunks is New York Dancewear.

I've only made a built-in bodysuit once. One of the things I learned was that, from the waistline up, the bodysuit needs to be the same size and shape as the dress from the waistline up. From the waistline down, I use the Kwik-Sew master leotard pattern. So, to make the bodysuit what you should do is trace the upper part onto tracing paper, and then trace the bottom part, using a dressmaker's curve and some guesstimation to graft the two parts together.

It is highly suggested to cut out and sew the bodysuit before even starting on the rest of the dress. This will give you a chance to "test-fit" the bodice via the bodysuit. Any alterations that you have to do on the bodysuit to get it to fit you will also need to do on the dress bodice itself. If one layer is much bigger or smaller than the other then the dress will not hang properly.

After you've finished the bodysuit, make the dress, test both again for fit, and then sew the bodysuit into the dress. Part of why I avoid bodysuits is because I haven't quite figured out how to attach them to dresses that have sleeves, unless the sleeves and very top of the bodice are of a different fabric than the torso. When using different fabrics, such as mesh sleeves, you conveniently have a seam going all the way around the garment that avoids the issue of the bodysuit's arm openings. In other words, you've got a strapless bodysuit that you're going to sew into a strapless dress, but then the whole thing is going to be finished off with mesh at the top and mesh sleeves.

Sorry I'm not more help with this right now, but when I finally get one to work the way I want it to, then I'll be able to write a better description of what goes on.