To sign hurt, take your two hands and make each into a fist with your index finger extended. Point the index fingers of each hand at one another and twist the two hands back and forth in opposite pivots. Now if only there was a sign for hurt that adults could use with one another.
The sign is placed at the location of the hurt.
When a child visibly hurts themselves, make the sign over the location of the hurt. You can also do hurt with picture books where a character has perhaps a bandaged paw. Hurt is a tremendously useful sign. Funny enough, when a child is hurt, just being able to communicate to you that they are hurt and to have you understand that is worth as much as any medicine.
Hurt is a really useful sign to teach because it can really make both of your lives a lot easier. If your baby can communicate to you when she gets hurt, it allows her to share her hurt with you. You may not be able to do anything lots of the time, but it is often surprising how better it feels for your baby if you just give their hurt knee a kiss, a little rub, and a little empathy.