It looks like you're been knitting the top of the dress sideways, always slipping the first st of the row purlwise. This gives you a nice edge that has a chain of sts along it. You're now going to pick up in the chain, one st in each side of the chain st (one in the front and one in the back loop of the chain). When you get to the garter st button band, overlap the two bands and pick up 4sts through both thicknesses. This will stitch the bands together at the same time as you pick up sts. If you need to see a video of picking up sts there's one under the Free Videos tab at the top of the page, Tips, Picking Up Sts. And here's
a tutorial that shows you picking up in one loop of the chain. You just pick up in both.
Good luck with the dress. It's a very interesting pattern.