I picked my fourth frame to start with, however, because it's different enough from my first frame to be a good point of progress, but not so advanced that I can't eyeball the two in between to estimate just how far each segment of limb should have moved between first and second, and third and fourth.
Again, I used the Preston-Blair demonstration as a reference, and on my fourth frame (Legs layer) I drew my legs--with the supporting leg almost fully straight, and the traveling leg slightly upraised. I didn't completely straighten the supporting leg, although some choose to; this is just a personal preference, as I don't know about you but I can't completely thrust my leg out in a straight piston while walking without locking my knees rather painfully. For exaggerated marches and other flamboyant walk cycles, however, emphasizing a straightened leg can add to the effect.
