Preheat your oven to whatever your brownie package recommends (350 degrees is usually what they say)
Prepare brownie batter as per the directions on the brownie box.
Prepare baking pan: either spray the baking pan with cooking spray or line it with parchment cooking paper, which makes for easy clean up.
Pour the batter into the prepared pan.
Bake for 20-27 minutes, depending on the box recommendations. A 9x13 pan usually needs 20-22 minutes, a 9x9 pan is about 25-27 minutes.
Test for doneness by inserting a toothpick or butter knife into the center of the brownies. It should come out with moist crumbs stuck to it, not wet batter.
Remove the baking pan from the oven, and set the oven onto Broil.
Place marshmallows onto the brownies in the baking pan.
Put baking pan back into the broiler / oven for 1-1.5 minutes. Watch it carefully so it doesn't burn. You just want the marshmallows to get golden brown.
Remove baking pan from the oven and turn off the broiler.
Drizzle the warmed fudge sauce over the top of the marshmallows.
Add the graham crackers the way you like: break them in half and stick them into the marshmallows, or crushed the crackers and sprinkle onto the warm marshmallows, or do both.
Serve while still warm.