press the folded edge under first.
stitch as though you were doing applique. teeny tiny stitches, bringing your needle straight up through the layers, catching only two threads of the binding, then needle straight down through the layers.
or stitch in such a way that you catch only the top and the edge of binding that's been folded under. sounds a lot harder than it is. there are a couple of ways to do it, but neither is very easy to describe. experiment with some scraps until you find a method that works for you. it won't take long. just trust yourself and dive in. :P