You will need marshmallows, chocolate and graham crackers, and if you don't have graham crackers some digestive biscuits will do.
If you have never roasted a marshmallow, then you will have to learn how to do that first. You will need a stick for that. Something fairly straight. Insert your stick into one end of your marshmallow and roast it over campfire coals until it becomes toasty golden and melty warm.
While your roasted marshmallow is still hot, place a piece of chocolate on your graham cracker, cover the chocolate with the hot roasted marshmallow and top with another graham cracker. It is sort of like a candy and marshmallow sandwich.
And of course, the name comes from the fact that one is never enough and you will need s'more.