First take a piece of fabric, the inner "circle" should be the same as the neck's (when laid face sides together!).
This isn't very exact, cutting these wasn't my business:-P
Than sew it 3-6mm from the edge, according to the sort of fabric. Face to face!
Fold the piece to the wrong side and sew as far from the inner edge as you wish.
I sewed one more line. Cut the piece and zig-zag it.
Uf, I already have three costumes completed ;-)
