Here’s a brief tutorial from Karen Winters (www.karenwinters.com/kblog). She sketched the tree in pencil (while her hubby queue for coffee, hence the title of her post) then digitially painted it in layers using Photoshop.
I’ve created a grad from green to brown, pulled back the opacity to 50%, and set the grad to multiply…
… I made a composite of the previous layers and put it on a fresh layer. Then I added new layers and painted some foliage where it made sense. There are three layers of leaves, in varying opacity and color, to suggest volume and depth. I also painted a little color on the trunk.
Check out her blog post for details on how it was done.