First, you want to do the marshmallow as they will stay fresh for a few weeks. The day you want to do the s’more, do the crackers.
What I did is tempered some chocolate and dip one side of each cracker in. Afterward, I chopped in half a marshmallow and put the 2 pieces on it. Using a culinary torch, I grilled the marshmallow then quickly put another layer of cracker on top ( chocolate face in ) and press it down gently to mush the marshmallow a bit. When ready to serve, using the torch again, grill some more on the side of the marshmallow.
Obviously, if you have a campfire going its less hassle 😉