According to Angry Animator The most important frame in a walk cycle is the one where the heel of the foot touches the ground. Others may disagree, but if you don't get this frame just right, your whole animation will be off, and it will be very difficult to draw the recoil frame and others.
Also notice that the head doesn't stay in the exact same spot, but rather makes a curving line, determined by the intensity of the walk.
Of course, ol' Preston didn't give us a completely fluid cycle in his example. In order for it to loop smoothly, one would simply find the frame in the middle that resembles the first, then animate the previous frames only for the other legs/arms.
The rest should be self-explanatory. However, if you find yourself lost, here is a more in-depth tutorial
I have certainly learned a lot from just studying this image, I'm still learning, and I will continue to learn probably forever. Even the animators at Disney had more to learn once Pixar came into the picture! Now that I look at this again, It seems I need to practice my walk cycles now.
