I've found an interesting way of practicing with my sons help.
We went out to our local woods, and found as many different trees as we could. Carefully detatching each twig from its tree. We identified each one using 'Collins Gem Trees', Then labelled them with a piece of numbered tape.
When we arrived home with a bunch of twigs, we placed them into individual jars, tubs or bottles with enough water to cover the bottom 30mm of twig. Sited them on a warmish window sill, with good light.
They have been there for 6 days now, and have just started to show emerging leaves. Both me and my son, are seeing the leaves develop, and will find out if we guessed right on their species; which is helpful as each have their own characteristics. So next spring, we should have a better idea of different species throughout spring.
I've enjoyed it, my son loves it, and it is better than anything a book can show you of a developing leaf/ twig. And when the leaf is fully developed I'll realise he's done much better than me guessing.