The trick to the trick is:

1. Tell your puppy to sit.
2. Put a treat in your hand so the dog can see it. If the dog stands up, when it sees the treat, get it to sit and remain sitting.
3. Slowly go with the treat around the left side of their face. As they try to maintain eye contact with the treat, they will begin to get up and follow it.
4. Give the command "turn".
5. Assuming the dog completes the turn, reward with the treat.

The hardest part is to keep them sitting when they first see the treat.

Bonus: The roll over is the same trick! The only difference is that they start out in the laying position. In fact it may be better to teach them to roll over first. As when they attempt to follow the treat with their eyes they will flip on over. Given the down position is the most neutral posture, it's easier in some way to teach them this trick than the turn.

If they master the trick with the treat showing, then the next step is to stand a few feet away and do give the command with a circular hand motion. My Daisy dog that I recently lost after 12 years, would respond to hand signals(no voice command) from a distance of 20 ft to turn or roll over.