Take your solar oven out into the sun and place it so that the sun shines directly inside. If your lid has trouble staying open, use a straw and some tape to prop the lid open.
Now let your s'mores heat up! The cook time will depend on the temperature and the amount of sunlight your oven gets. In the hot California sun, it only took about 20 minutes. It might take longer, but be patient and check back often on your s'mores. Every time you check back, you should see them get a little more melted.