When shading an object with a pen, you cannot manipulate (change the color like a lead pencil). Then it becomes even more important to be very intentional with your direction of lines.
The best way to get shape and shading in a sharpie drawing is to do crosshatching. (Using lines to create a 3d figure).
For this demonstration, I will be drawing a tree using the cross hatch method.
Draw a basic outline. Don't worry about shading just yet.
Be very intentional will the direction of your lines. Round the lines around branches.
Simultaneously shade and create bark texture.
Cross hatching doesn't have to be straight lines! Curve your lines to make your drawing 3 dimensional.
Cross hatching becomes more apparent at the top of the tree ( where the most shading happens).
It's okay to turn your paper.
Now it time to draw the leaves. You won't use as much cross hatching in the design of the leaves.
Add some random leaves. These will be he most pronounced.
Draw a basic outline of where you want the leaves to be.
Now crosshatching at its best. Use this in the inner parts of the tree.
Continue adjusting the shape of the leaves.
Continue crosshatching until you've finished the leaves and drawn a shadow. Make sure to sign your art.