To sign circle, start with your index finger of your non-dominant hand pointing forward in front of your body. Then take your dominant hand index finger and make a circle, starting and ending at the stationary index finger of the non-dominant hand.
Shapes like circle, square, and triangle are good intermediate signs. Introduce a few shapes at the same time. The contrast between the shapes helps to teach baby the concepts more easily.
The flash cards a are a great medium to teach the basic shapes to baby. Once they learn to make the association between circle and the flash card diagram, try and help them broaden their understanding of the sign by finding circles in your environment. For example, they may see circles on the wheels of a car.