Then you're going to turn on onion-skinning, so that you can see where your frames are in reference to each other, and space out your duplicate bodies across the keyframes so that they seem to move in an up-and-down wave, following the path of motion demonstrated by the dotted line in the Preston-Blair example.
The reason for this is because when we--or any creatures--walk, we don't travel exactly in a straight path. As our legs bend and straighten, and our feet extend, flatten, and push off from the ground, we're going to be propelled upwards only to sink down again. When walking we're never the exact same height as we might be in a resting position, save for in a single instant of motion as we pass through that particular plane of space.
On my first frame, my stick-man is near a mid-point; he's not as tall as he might be when his supporting leg is almost fully straight, but he's not as short as he would be with his supporting leg fully bent, either.
