The tunnel is an easy, simple step. You're going to use part of the basket step, which requires to lift the left hand. Turn to follow. Place her on your right side, leaders. Just lift the hand high enough that she can go under. As you see, her hands are twisted. To untwist, just give her another turn to open.
This step can also be done by the leader. Really simple. Roll yourself in and go under. Either way, you can turn yourself or turn her. That's a tunnel step. Now if you want, let's try it with the music. Now the leaders. I could turn. Back together. That's your tunnel step.