placed the 4 hot rocks on the fire pit near the tree with the hard, yellow fruit? After that, you'll need to cover the fire pit with banana leaves, to retain the heat. There's a banana plant to the right of the blackberries. Drop a villager at the base of the banana plant and they should pick up some banana leaves and take them to the fire pit. Once the pit is properly covered, drop a villager at the base of one of the trees with the hard, yellow fruit and they'll do the rest.