To make Roasted Marshmallows, just go to Bellflower Meadow and click the fire pit in front of the Log Cabin. There you will see what’s required: 5 Fir Logs, 4 Bush Wood (cut wild bushes), and 1 Marshmallows (make it in your Farm Kitchen).
You can carry the Marshmallows you made in your Farm Kitchen to BfM with the truck.