Knowing how to draw a circle is a basic skills for artists. For animators - it's a must.
Practically every animated character starts with a circle, so let's get down to business:
The only joint in your arm that can make a full, smooth circular motion is your shoulder.
It's a ball-and-socket joint, and the first step in drawing nice round circles is learning how to draw from the shoulder.
Find a large whiteboard, or the biggest sheet of paper you can get. That would be a full A1 paper, and mount it up.
Stand, don't sit.
Warm up your shoulders for a few minutes. Roll your head, then your shoulders, then your whole arms.
Now take a marker or a pencil and draw the largest circles you can make, using your whole arm.
Practice drawing circles in both directions - clockwise and counterclockwise. Again, and again!
When you have gotten used to the feeling of using your whole arm, and drawing from your shoulder, move on to a regular sized paper (A4) or your wacom tablet.
Now, on the smaller scale - practice drawing from your shoulder. Don't lean or rest your hand on the table. Make your arm float over the surface, even if you are drawing very small circles.
Watch the video further down this page to see a great demonstration on how to draw circles like a pro.
Download these Circle Practice Cheat Sheets, and practice tracing the light gray circles. (hover over the image - right click - save image as).
Once you manage to make more or less decent circles when tracing, try freehand.
Practice drawing figure eights, back and forth, in both directions. Here too, draw on the largest paper or board that you can find. Move your whole arm, from the shoulder.
You will find, as most people do, that one side of your 8 is rounder and smoother than the other, which is flattened or squished ,or stretched.
Work on the less-round side. Make a conscious effort to smooth the curves out, make it nice, round and consistent.
Mickey Mouse is made up from circles and curves.
Here's a video of Disney Animator Michael Surrey showing us how to draw Mickey Mouse.
Pay attention to how his hands hover over the page, how he practices the circular motion before putting the pencil to the paper:
Now that was old school classic animation :-)
Let's move on and see how Animator Eric Goldberg draws curves on a Wacom Cintiq.