There are two ways to approach a walk cycle.
You can draw it with your character standing in place, or with the character moving across the screen (or page, but this is a flash tutorial, so I'll stick with screen).
Making a walk cycle with character staying in place is a TERRIBLE way to do a walk cycle. It might look okay when it's on that invisible treadmill, but if you want the character to move across the screen, either the feet are going to slide on the ground (a huge pet peeve of mine), or, if you plant the feet, the movement of the character will be jerky. I worked with a guy who spent a loooong time making his walk cycle "perfect" but once he got the character off that treadmill, and tried planting the feet, it was a disaster. And I've certainly been guilty of this mistake myself. Here is my first walk cycle I ever did in my life (for an introduction to Flash class in college). The design is so ugly that I'm only showing you the ugly hips and legs.
Here is the walk cycle on the treadmill.
And here is it moving across the screen, with his feet planted.
Yuck. When you walk you have a consistent speed, but this guy's pace is all over the place.
The right way to animate a walk cycle is to have the character moving in space. Working this way, it's easy to keep the feet planted and the speed consistent. But a lot of people avoid this method because it's hard to keep a character's volumes consistent when it's moving through space.
But using Flash, I came up with a way to combine the two techniques, and get the best of both worlds. For simplicity's sake, I'm going to do a super boring, torso and legs only, Richard Williams walk cycle, but once you get this technique down, you can alter it and get interesting walks from it as well.
First, draw your torso and head.




I start by drawing the legs for the first contact pose on the sketch layer. This pose is the same as the pose for frame 17, so I copy the frame, paste it on frame 17, and line it up with the torso there.





Not the most exciting walk, but you get the idea. Next comes the inbetweens. Let's start with the the up positions on frames 7 and 15. Make your motion tweens, key out the walk cycle symbol at frames 7 and 15. Remove the tweens, go inside the walk cycle symbol and adjust the torso and head to be at their highest position. Then go out on the stage and sketch your legs.


Okay so now we have the right leg positions for our walk. But chances are, your leg volumes are inconsistent (mine sure are). To fix this, we do our leg clean ups INSIDE the walk symbol. This way we use our original sketch as reference for the correct foot position, but when when we're inside the symbol, our torso isn't moving, so it much easier to compare the volumes of each leg. This is basically the Flash version of a traditional animation technique, which was explained really well by the disgustingly young and talented Matt Williames on his blog Hand Drawn Nomad Zone. (working professionally for Warner Brothers at age 14????)
So go inside your walk cycle symbol, and create two new layers, one for each leg. Then at frame one, go inside your symbol and draw your cleaned up leg. Move to frame 3 on the stage, go inside the symbol and clean up those legs etc etc. I cleaned up one leg at a time, and worked straight ahead. This helped me keep track of things like the spot where the leg connects to the hip, and the angle of the thighs and shins. I would also start the clean drawing with the rough visible, but then turn off the rough to finish the clean drawing on it's own.

When you're done, you'll have a walk that moves through space, but also one that can be stationary, if you keep the symbol stationary. Here's my "final". For yours, I would recommend at least adding arms and a head bounce. And if you're really good, you'll also redraw the torso and hips so that they twist in space and don't look like the super lame stiff animation that gives Flash it's bad reputation.
Now you probably want more than two steps out of your cycle. To do this, on the stage, copy your frames, and on a new layer paste them starting on frame 17. Then using the edit multiple frames option, move these newly pasted frames so that they start where the bottom layer stops.


Again, this walk cycle is REALLY boring, I know. But you can do more interesting ones using the same basic technique. Here's one I did last year.
And once more, I am now offering this tutorial's FLA for 3 dollars through PayPal. Currently it's only available for Flash CS3 and up, but if there's demand I can make it for other versions as well. The FLA includes the walk cycle symbol, the leg sketch layers and the elf walk cycle.