I don't know of any specific guides for pixel art walks. If you want to learn how to do walks, my biggest suggestion for resources would be The Animator's Survival Kit by Richard Williams. The book is one of the best technical guides for animators. He talks a lot about how to approach thinking about doing walks, along with other physical performances. The other biggest resource would be your own time and dedication, and applying that to your art as much as you can.
Animation isn't one of those things that you can do well going from how-to's and guides alone. They'll get you started with the basic process, but to make it really pop, you have to understand both the mechanics (how) and psychology (why) of the movement. The only way to get to that point is to animate and think about animating.
That said, I'll try to break down the way I do walks.
I start by blocking in the extreme key frames (or just "extremes" for short). These are the absolutely crucial images which form the foundation of the animation. For a walk, these would be the stride and passing poses. The stride pose is when one foot is farthest out front, and the other is farthest out back. The passing pose is when one leg is directly under the body, and the other is lifted on its way forward. These poses generally define the highest/lowest bounds of how much the torso will travel up and down. At this point, if I'm just doing a mechanical walk (as opposed to an acted or "character" walk), I'll only block in the basic body shape and the legs. For an 8-frame walk, these would be positioned like this: [Frame 1: Passing, Frame 3: Stride, Frame 5: Passing, Frame 7: Stride] (The strides are not at 4 and 8 for a reason I'll explain shortly). Just getting the extremes down means I've roughed in half my animation already!
I then block in the Breakdown keys. These are also important story-telling frames, but they are not as foundationally necessary as the extremes. In something like an 8-frame walk, all of the frames end up being breakdown keys. Frames 4 and 8 form the Down position keys. When you walk, the stride is where you reach out to begin to catch yourself from falling. The "Down" position is the result of you catching yourself as you fall. The front leg goes from relatively straight to bending a bit as you transfer your weight onto it. Frames 2 and 6 don't have a specific name, but they are the keys where you determine how high the forward-traveling foot will lift before being planted in the Stride frame.
If you were doing more than 8-frames for your animation, you'd then move on to your in-betweens, which are just the filler drawings to complete the smooth motion from one key to the next. For simple or very mechanical motion, you may have one key every 6-12 frames, with a lot of inbetweens. For more complicated motion, you may end up having only a few, or even NO, in-betweens.
I've been glossing over it a bit, but as you're doing this, you should be making sure that the torso is moving up and down smoothly b/w the high and low points. In something as low-rez as the walk you're doing here, a 1-pixel shift is all you need.
So far, I've covered everything to get to where my paintover finishes. Once the basic torso and legs are done, you can go in and layer on torso twist, arm swings, head bobs, whatever. If you keep up the same Extremes/Breakdowns/InBetweens workflow, you should be able to tackle things more easily b/c you're breaking them down into manageable chunks instead of trying to do it all at once.
As a side note: don't animate with "finished" art. As you can see with my paintover, I'm just using broad shapes to get the motion down. Once everything is moving nicely, then you can go back, clean it up, and make it pretty. Trying to animate with full outlines/colors/shading just means you have to scrap more work when you need to fix it.
I've kinda trailed off b/c it's late and my brain is frying, so if any of this doesn't make sense, just ask and I'll post a response sometime tomorrow.