Fear is signed exactly like afraid. It starts with your open hands (or closed fists) by your sides, with the dominant hand around chest level and the non-dominant hand around stomach level. Then you bring your hands inward, in front of your body, palms facing inward and fingers spread in an open five hand.
The same sign is used in Baby Sign Language (and ASL) for scary, and frightened. And remember, when doing any of these signs, emphasize the emotion with your facial expression.
Introduce fear when you notice your toddler in a situation that is causing them to be fearful. Because these tend to be highly memorable situations for your child, they tend to make the connection to the sign quickly. Fear is a useful sign for toddlers. It lets them communicate to you what events cause them concern. The things causing your child distress can often be things we find innocuous, such as a blaring ambulance siren. And once you know that something is causing your child concern, it is usually easy to either avoid it or to help them overcome their fear.
Teach your toddler how to sign fear when something causes them concern or anxiety, or when they recognize the same facial expressions in their storybook or cartoon characters. Signing fear can help your child avoid the cause of fear or overcome the fear, simply by communicating their feelings to you.