To make small, round shapes out of cardboard use an office hole punch (available also with funny hole shapes nowadays) or better still a DIY punching tool used to make holes in leather belts and the like »
The trick is to think backward. Normally you paint shadows in the recesses of a model and then highlights on the edges emphasising how the external light plays on the shape. Fire has a light of its own and so the roles are reversed and the deeper you go, the lighter it will be. »