You just take the chocolate and the graham cracker and make a sandwich with the marshmallow, like this.

[He's getting out a piece of chocolate and a graham cracker as he says this. First he breaks the graham cracker in half, then places the piece of chocolate on the bottom half, then the marshmallow on top. Finally, he puts the other half of the graham cracker atop it like a miniature sandwich and holds it up.]

See? The chocolate will melt a little since the marshmallow is so warm, so it gets all nice and gooey.