umm it's kind of a long process xD but in the most basic form
I draw the silhouette of the tree with flat colors first
as for the trunk, I use a soft darker tone to paint the base of the details
and then refine the details with even a darker tone to add more depth to the initial details, and later add some highlights with a lighter tone
as for the leaves, it's pretty much the same process
use a soft dark tone for the base of the details, and refine with a darker tone and add highlights
I tend to draw each leaf one by one manually

this is kind of an old example but still works uvu-b