I decided to draw a tree tutorial for you guys, since I figured out a cool way to make them. ^.^
1. Pick a brown and draw a tree trunk.
2. Pick a green and draw some lines going from the trunk outward. I used pen pressure and brush size 6.
3. Using a darker green, build the shadows in the tree.
4. Take a lighter green and draw some highlights. I used a size 3 for this.
5. Use the smudge tool and smudge the edges of the tree outwards. Also add some darks and lights to the tree trunk.
6. You can use this panel and try it yourself! ^_^

