For a walk to count, open the Walking tab and tap Start Walk, then carry your phone with you as you go. Keep walking until the progress bar in the app shows that you've completed the walk. It's okay to take pauses during the walk, but remember that your progress may reset if too much time passes without movement.
You'll need to wait at least one hour before another walk can count toward your daily walking points. The Walking tab shows how long you need to wait before your next eligible walk.