Both is signed by first making the number 2 hand, then pulling it down into your non-dominant hand formed in the shape of a receptacle (or a horizontal letter C hand), while drawing the index and middle fingers together at the same time the hand is entering the "receptacle". It is like shooting two basketballs into one hoop.
Teach your child the both sign in the context of paired people or objects, when talking about both children getting ready for bed, both you and your spouse doing something special for your toddler, both grandparents planning a visit, and so on.
Teach your child this sign when the occasion calls for it. It is also good to have the foundation of the numbers flash cards already before teaching both.