1st-Watercolor two pages, one with grass and sky (however you like, be creative), and the other page with various fall colors (for the leaves). You can use white construction paper or watercolor paper. Let it dry overnight.
2nd- Draw and then cut out your tree, either black (for higher contrast) or brown (realistic). I made this example for the kids at my daughter's 2nd grade class, and the majority of the kids chose to make a brown tree.
3rd- Then cut the Fall colors page into triangle-shaped pieces for the leaves and glue/paste onto the other page. You can get creative and involve wind blowing them around... falling off the tree... piling up so high... whatever you like!
Unfortunately I don't have any of the kids' creations right now because it snowed the day we were going to finish them and they closed school for the following 3 days! Hopefully they will finish them in class and I can take pictures of them. The trees were so wild and fun!
Whenever I see projects for kids, I always to try to change them up a little bit. I found this project on this amazing website for children's art