Frightened is signed exactly like afraid or scared: 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 number 5 hand.
Frightened is a useful sign for intermediate babies. It lets them communicate to you what events cause them concern. The things causing your baby distress can often be things we find innocuous, such as a loud blender. And once you know that something is causing your baby concern, it is usually easy to either avoid it or to help your baby overcome their fear. Often just naming the emotion does a lot to reduce their anxiety.
Teach your baby how to sign frightened when something causes them concern or anxiety. Signing frightened can help your baby avoid the cause of fear or overcome the fear, simply by communicating their feelings to Mom or Dad.