Here is how I made each layer:
Leaf Shadow:
Use the leaf brush. with both foreground and background colour set to black. Paint wherever you want there to be shadow, and make sure its on a seperate layer to everything else. Once you're happy, on the layers box, select the leaf layer, and where it say opacity on the layers box set that to anything you like - I think it was a bout 30 on mine. This will add a nice shadow over anything beneath it, creating the illusion of a leaf canopy.
Tree trunk:
Using a preset, 100% hardness brush (one of the first ones), paint a tree shape. Neaten it out, so it looks good. Now, you are going to lock the layers transparency. You may have noticed on the layer box, the word "Lock:" followed by several symbols. CLick the first one, the one that looks like a tiny gray and white checkered box. You should notice that you can now no longer paint over transparent areas. This means you can recolour entire layers without going over what you have already drawn.
Now, add areas of shadow and highlights. Don't worry about being neat, just splodge the colour on. Once you're happy you have the shadow and highlight colours in the right place, use the smudge tool on 50% strength to blend the colours together. Once satisfied, move onto the final layer.
Grass:
By far the easiest. Choose a nice yellowy dark green. Paint the entire layer that colour. Now, as we did for the tree layer, add colours of varying greens and yellows, and smudge them together. Maybe have browns beneath the tree trunk, like in real life where grass has died away.