On cardstock, practice these techniques by drawing 4 circles using a circle template. Create a very light pencil outline, (that you will eventually erase.) Leave room for the shadow as you draw your circles, and then practice these 4 techniques with the goal of creating volume by using dark to light values. Use a ultra fine point sharpie or better yet, a 05 micro point pen.
Here are some good examples of pen and ink illustration techniques.
Edward Steed is one of my favorite illustrators. I love his funny characters and his imaginative combinations of texture, form and color. I like his work because it inspires my own doodling. I recommend doodling anytime you are in a place you don't want to be, like a staff meeting, or while you are learning in other classes. The doodles can help you remember what the teacher was saying at the time. All of these copyrighted images are from the site link below.