I started this drawing quite some time ago, but didn't like the way it was going. I put it away and decided last night to bring it back out and work on it again. In the past, I would have left it unshaded, but recently I've been working on my shading.
Even though I like the drawing unshaded, it looks so much more complete by just adding a bit here and there.
It's as simple as laying down a line with the flat edge of a pencil and smudging it with a tortillion in the areas you want shading. Since this is zentangle, and there is no right or wrong, forget light sources. Shade the edges you want to be darker. You will notice in a couple spots I used a white gelly roll to add some highlights. There weren't many places to do this since I used a white sheet of paper as my base, but I used some highlights where I could and it really makes a difference in the overall appearance. Shading really does make a difference.