When I discuss the geologic feature, I have a model that can be clearly seen from the back of the room. Just be sure to lock the wheels on the overhead projector cart. Someone is bound to bump into the cart and mess up your drawing. Take a look at these step by step instructions . Instead of using an X-acto knife, try using a pin to poke through the cardboard along the contour lines.
Some teachers say it helps the students when they cut along the lines.
Be sure to cut the rectangular base first. Use a metal straight edge when cutting the sides of the model. Strips of aluminum are readily available at hardware stores. These can be easily cut with a hacksaw. As your students cut the layers, have them number each one with the correct contour interval.