Elizabeth, most clothing patterns use a 5/8" seam allowance. Your collar will need a lining, like white cotton, and after you sew your 5/8 seam around the outer edge of the collar, it will make the inner edge much smaller, and it should fit around the neck just perfect then. Your neck will need a binding or a facing on it too. I dont do things the proper way. This is how i do them. Do your zipper first, get it done and over with, then sew up the rest of the centre back seam 5/8 seams. Then do your shoulder seams. Then gather your sleeves with a loose running stitch, and pin to sleeven. Now sew the sleeve in. Now hem your sleeves, now sew your side seams. Now do your collar. I know, i do things in the wrong order, but easier lol, now hem the dress :)