Your batting should be squared off with the top. Fold the backing over at that edge to the front. You are going to turn under about 1/4 inch to hide the raw edges. Take a look at how much binding shows. If it is too much, then trim it down to the width that looks best. I usually let around 3/4" show on top.
You can pin it, or baste it. To make it look nicer on the top, try using one of the fancy stitches on your machine. This serves a dual purpose because it secures the raw edges better.