Round is signed like circle, but minus the non-dominant index finger acting as a reference point for where the round shape starts and ends. Simply take your dominant hand's index finger and make a circle, starting and ending at the same imaginary point.
Teach your baby the round sign when you encounter balls, round lollipops, bracelets and rings, and all sorts of things that have a circular shape. 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 your baby. Eventually, they will learn to make the association between the concept of round and the many objects in and outside the home. For example, they may see that their big pancakes are round, just like their smaller bracelets or bangles.