I started with a piece of scrapbook paper. I used paper that was textured so that it would look a little nicer than plain paper (but any paper will work). Next, I took brown paint, mixed in some metallic brown for fun, and painted a tree on the paper. Then once dry, I gave each kid a painted tree and some buttons and had them glue the buttons on using clear-drying glue and a paint brush. They had glue all over the buttons and paper, so its important to use a clear-drying glue, like Elmer's. Since we were doing fall pictures, I grabbed bags of buttons that were assorted fall colors. But, you could also do a nice spring picture by using assorted green buttons and maybe even a red here and there for an apple or a pink for blooms.
Simple. Beautiful.