I really like this exercise because it is easy & fun and you can incorporate it in many different situations. 

The goal of the trick is to have your dog go around you and between your legs. And then to have them between your legs as you move forward, until they hear the release word. 

We are going to start teaching this trick by having multiple treats in each hand. Start off with your dog directly in front of you. Show him one of your hands and slowly lure him from in front to behind your body.
As he gets about halfway around you, put your other hand between your legs and help him get into position. The moment he gets in position, start giving him a steady stream of treats. After a few treats, you can say your release word and throw a treat forward to release him away. 

We want to keep doing this until your dog is confidently and fluently doing so. 

Once it is going well, we can start adding some movement to the exercise. Guide your dog into position, give him a few treats and then as he’s following the treats in your hand, take a few steps forward. As he follows along, give him lots of yummy treats. We can continue to practice this step until your dog is confidently walking between your legs. 

Now that your dog has a good understanding of the trick, we can start adding a verbal cue. I use the cue “Between” but you can use whatever you like, as long as you are consistent with it. 

To add the verbal cue, we will want to say the cue and then help with your hand motion.
It will look like this: “Between”, then motion. Moment the dog gets in position, lots of yummy treats.