I don't make dog dresses--Snoopy is a boy...
But I do sew, and I would stitch the ribbon down on each edge. For the bow, try making a big loop that will be for both sides, put a little piece around the middle, and tack the ends together. If the loops part is a little gathered, it will look more like it's actually tied. Then add a piece folded in half for the hanging down part. Sew the whole thing to the dress. I hope that's not too confusing!