Hmmm….I think the easiest way to do this is to sew up to the point where you want your buttons to begin. Press the remaining part of the seam so that the raw edges and the seamline is all the same, even though this part is open. On one side sew on the buttons of your choosing. On the other side, sew on some button loops. I would have the raw edges of the button loops on the wrong side of the dress and then maybe cover this section with some thin grossgrain ribbon just to make it look nice and extra secure on the inside of the dress. I hope this works for you.