There are several types of cut. First option: make casting a wedge in the back seam. The step length will increase. The length of the wedge must be greater than the length of the main skirt to the length of the train. The bottom make smooth, rounded line.
Instead, you can extend the rear Central seam at the required length. Do not forget the pattern to take into account the allowance for processing (1-2 cm). Similarly lengthen and widen the side seams. The bottom line also needs to be rounded, but in the simulation it is possible to encapsulate it in a obtuse triangle, or another shape, depending on the model of the dress.
To the bottom of a very long loop, you can sew a ring that you can wear on your finger. In this case, if you want to dance, you can clean up the plume. It will be beautiful flutter in the whirling.