I have those wonderful tree pictures and decided to do a large doodle about them.
1. Use a pencil to create a simple sketch of the tree.
2. Go over all lines with a black Sharpie marker
3. Add a horizon line and some smaller trees in the background. This will give you some visual depth and interest.
5. Now you're going to work in the negative space around the big tree. First I used a light blue marker in the area that would be the sky.
6. Then I used a dark blue marker for the the background trees. Notice I used straight parallel lines (hatched) for the sky and curved ones for the tree trunks.
7. For the ground I used wavy parallel lines in green.