Ooh - a good bustle can be fun! My experience is that there is no REAL scientific method to creating a beautiful bustle - well, there is SOME method to the madness, but it's harder to calculate. I'd say it falls in the category of draping patterns vs. drafting patterns from slopers. It's more like draping.
The point is, it takes a lot of playing with the hem and train to get it to look just right. Here are a few I have done that I have pictures of...
The purpose of a bustle is to bring the back hem and train up in the back of the dress so that the bride can walk, dance, not drag her dress, etc. These first two dresses are simple bustles. They are bustled "on top" of the dress. Meaning: the maid of honor - or whomever is the bride's helper - can bring the bustle up without having to get under the dress.
There is some type of hook and eye or loop and button nestled in the lace or on a seam that can be easily pulled up and attached. Fluff it out and Voila!
This last dress (above) had a cathedral train (about 10 feet long). It required a different kind of bustle. This is called a French bustle. Basically that means the dress train gets pulled "under", in sections, creating the puffy part on the top. All layers have to be pulled together underneath the dress. Yes, the bride's helpers actually have to lift it up and go under! There is usually a series of coordinating ribbons per layer and per point of contact that have to be manually tied. (It can get hot under there too!)
Then when you come out for air, it has to be fluffed just right. It's a real commitment and you better get all the pictures you want with it all tied up before it gets taken down again!