Tunnel is a natural sign. Form both hands into the ASL letter C sign, then hold one directly in front of the other, both hands touching and aligned at the center. Proceed to mimic the length of a tunnel as both 'C' hands move forward in unison, away from your body.
Teach your toddler the tunnel sign in context: when you introduce them to the different playground equipment including a tunnel, when you are riding a vehicle or train and you are passing through a tunnel underneath a mountainside, or when you are taking the subway.
This is a useful flash card taught best in context. Children get excited going through tunnels, as the mystery of not knowing where they will end up brings an element of adventure to any game or road trip.