Do it during lunch, after work, after dinner. On the weekend , for a longer walk. Parks and trails work best. Less crowded streets in the city work as well.
Don't rush, just walk slowly.
Observe things.
If you end up thinking about the problem your are working on, that's fine, if you notice a bird's nest, that's fine too.
Walk like you have no place to be, look around. You will feel weird at first, but that's fine. Try not to care about that.
Think of it as something between meditation, relaxation and exercise. Sort of a all-in-one.
It's easy. There is nothing you have to do to plan for it, no special gear to buy, all you need to do is to say "I want to take a walk" and then ... take a walk.