cut the fabric this way (2 pieces) after making your dress ready to fit. measure the edge perfectly and give half a cm extra for folding.
now sandwich as - 2 layers of cloth facing each other and one collar stiff material.
this is how it looks like.
after stitching, turn them inside out. and it looks this way. sew on them to give a perfect look.
leave a cm of cloth not stitched at the 2 ends to fold into the main dress.
cut the white fabric that are popping outside, evenly.