Flash Frame-By-Frame Animation: 8-Frame Basic Walk Cycle

From Adrien-Luc Sanders, former About.com Guide

If you've got those first four out of the way, you should be just fine doing the next four as the upright step turns into a mild forward lunge into the next step; use the Preston-Blair reference for the fourth and eighth frames, and then use your own eyes and reasoning to work out the frames in between. Your end result will come out looking like a depiction of the evolution of man, but it should portray a single full step.

One thing you need to remember about this sort of motion is that you should never really be thinking in straight lines. If you observe the way the legs move, they don't scissor back and forth on vertical paths of motion; they rotate at the joints. Almost all motion of a bipedal figure, even if it looks vertical, is actually taking place on an arc. Watch as the back leg lifts between frames two and three; it doesn't glide through the air diagonally in a straight line. Instead it pivots from the hip, while the knee traces an invisible arc of motion on the air. Try bending your leg at the knee and then lifting it up from the hip, and trace the path of motion of your knee with your eye; it will form a curve, rather than a straight line.

You can see it more clearly if you raise your forearm straight before your face, with your hand palm inwards and flat; "chop" your hand to the side without twisting it, moving your forearm at the elbow, and the arc of motion that your fingertips trace will be easy to follow.