Bad starts with your flat, open dominant hand facing you and touching your chin. Then move your hand away from your face, flipping it so it ends up with your palm facing forward.
As this is a warning sign, be sure to show a serious or sad face while signing, to underscore to your baby that a particular action is not good.
Used sparingly, word signs of warning like bad, don't, and no are useful with older toddlers, to have them stop doing something without causing a fuss or needless embarrassment in front of their friends. The sign can be taught by incorporating it into story books as well as real-life experiences. “Isn’t the big wolf being bad?” or, "Did Paula do something bad during recess?"
Bad is a contrast sign paired with good, so it is useful to teach when the situation calls for it. Signing bad when necessary will reinforce to your baby that certain behaviors and situations are not acceptable, are dangerous, or can cause pain.