I teach them basic geometric shapes in 3-D, then do contour drawings and
expand it into a lesson on negative space. You can use an old frame and
section off part of the bicycle. Then have them use the 12X18 paper and have
them draw what's inside the frame. Ask them "what shape is in the upper right
hand corner?" -"How far is it from the middle?" Where does it connect with
the next shape?" etc.........take them step by step in the beginning.
Demonstrate how you would do it and then give them a problem to solve