The best way to do leaf rubbings is to use oil pastel crayons. They are like creamy crayons, and the rubbings come out much nicer than ordinary crayons. I always cut the paper in half so the children can do one leaf per page, labeling each kind of leaf after doing the rubbing.
Always choose darker colors for the crayons; the lighter colors don’t provide enough contrast. Hold the crayon sideways, parallel to the paper. In other words, use the side of the crayon. Make sure to get good coverage so that you can see the veins of the leaf. Pay attention to the edges of the leaf, to make sure the shape of the leaf is clear.
You can hole-punch the pages and make a book out of the leaf rubbings by adding a construction paper cover and binding it with yarn.
Another variation is to grab some black paper and do a leaf rubbing with a lighter-colored crayon. Yellow, light green, or light orange work well. The leaf rubbings come out looking gorgeous. And if you shine a black light on it, it will glow in the dark!