All you do is take about 16" (41 cm) of ribbon, and fold it in half, and then in half again. Stick a pushpin through the middle, and then into the top of a walnut. Voila! So easy and sweet.
At the museum they had painted their walnuts gold which was pretty, but for simplicity I left ours natural. I also tried it with pinecones and it works great as well. It might, however, be a good idea to put a little bit of white glue on the tack before pushing it into the nuts, as they can sometimes be quite loose.
Adelaide adores this activity, and so we've been making them together to hang on our tree. She selects and passes me the walnuts, and I add the ribbon. We choose a spot and hang them on our tree together.