This is a powerful tool that can be done anywhere. Traditionally a Labyrinth is used in which you wind your way to the center and then reverse your way back out.
I use a nature trail which consists of a loop which I reverse about half way. It's very intuitive but here are some phases I use.
1. Just start walking and tune into your body using a body scan. I start with my feet and work my way up to my head just observing my body. Transition the focus to the rhythm of your walk and finally to the breath.
2. Observe your thoughts and mentally set them aside on the trail. I use the motion of the walk to leave any negative thoughts behind.
3. Be present (in the NOW) and balance your external and internal attention. Observe (using all your senses) the physical location as well as your emotional state. I use affirmations at this point to direct my emotions and thoughts.
4. Welcome the thoughts that come up. I support with affirmations. End when you feel done.
I don't time myself but I consistently take 30 minutes. Your mileage may vary.
A good phrase your can repeat during the walk is:
“Yoga is a deep well into which we can tap drawing forth whatever we need to bring ourselves into wholeness and balance.”