Costume Questions

[raptorgirl.livejournal.com]

I'm working on my Halloween costume, and having some issues assembling it. If anyone has tips on either of these questions, I would really appreciate it:

a) When making a round collar or neckline, how can you get it to be perfectly curved? The neck of my dress came out kind of polyhedral.

b) When making a dress, what's the easiest way to sew the top and skirt together? I've never made a dress before, and I'm having issues figuring out the logistics of it.

Also, does spray paint adhere well to styrofoam, or should I put some primer on first?

Sorry for all the questions. If my costume comes out well, I promise I will post plenty of pictures!