For a single step, you should only cover one stride length in distance. You can take a simple measure of stride length by drawing a line on a new layer between the heel of the forward foot and the heel of the backwards foot at the point where they're the farthest apart; I have two stride lengths depicted, because the step starts off mid-stride where the extension is the greatest. The full eight frames, however, only move the figure's body over one stride length.
The easiest way to line them up properly is to use the feet. For the first four frames, even as the body travels forward, the forward foot remains planted in the same spot. You can line the heels up--and, as it starts to bend and lift, line the toes up so that although the upraised leg travels and the body moves forward, that single support point remains stable.
On the fifth frame, when the moving leg touches ground while the base leg leaves contact, you can switch feet and start lining up opposite foot on your shape. Basically you should always use the foot that's against the ground as your point of reference to make sure your frames overlap properly and your figure travels the correct distance.
