This is a tree from my upcoming garden picture. When I paint a tree I start with a very loose sketch drawing the trunk of the tree, the main branches and the silhouette of the foliage. No need to be precise, just get a form that looks good to you.
Then I grab a soft round brush and I paint the main colors. I try to add smaller group of shadows to the foliage so it looks more realistic. I think about where the light is coming from and I also know what kind of light that is. Here, we are after some rain, the sky started to get clear and the sun shines from above. The top part of the tree will get most of the lights and the ground reflects some light back too.
In the last step I shade the trunk and add more detail to the foliage. I paint individual leaves here and there. This adds some more realism to the tree. I use for this a round brush that has more hard edge to it, but still not a full hard edged brush.