Start by sitting everyone in a circle facing inwards. Each person should chose their own unique 'sign' which is a movement or motion such as touching their nose, brushing their hair, giving two a thumbs up etc. Go around the circle and give people the chance to demonstrate their sign to the rest of the group.
Choose someone to be the 'guesser'. Get them to close their eyes whilst you choose someone in the circle to start the game. The guesser can then open their eyes. This person you choose to start then has to secretly 'pass' the sign onto another player without being caught by the 'guesser'. To pass, you must make your sign followed by the sign of the player you are passing to.
The game continues until someone is caught, then they become the guesser.
For younger groups, you can make the "pass" simplier by making people just do their sign and look at the person they are passing to.