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Increasing Dexterity - Drawing the line you want
Another great way to practice circles and curves is this exercise from drawabox.com:
Step 1: Draw a square plane (no need to draw it with the diagonal/vertical lines inside)
Step 2: draw a circle that JUST touches the edge of the square, in one or two strokes. (No furry lines!)
Step 3: Do it with lots of different planes!
Student work from drawabox