Find a tree that interests you. … Do about four preliminary drawings. Use a soft pencil, charcoal or pen and ink. Keep building up on the basis of previous sketches.
The tree I studied was a silver birch on my son’s land. The instructions mention sketching the foliage of the tree. As it was late winter/very early spring, there was no foliage so I had to adapt those instructions. For example the second study is of the bark on the trunk.
While I was at my son’s, I sketched the bark of a number of trees on his land.