Red dress disasters

I'm working on a new costume, and I unfortunately made the dress too big.

It is at least two sizes too big, so I have put it on the dress form and am resizing it with pins. Does anyone have experience with this? I have been pinning all the seams except for the back seam (where the zipper goes), but is this the right way to do it? Should I instead only pin the side seams?

I also have a feeling that I will have to build a bra into this dress: never done that before so it should be exciting.