Kent . . . . different people do this different ways. I tell you how I was taught:

1. Decide which side of the panel will face in/down.
2. Plane that side as flat as you can by eye (you'll probably want to use a #5 for this - you don't want to remove a lot of material here; just get it flat).
3. Lay the planed panel (planed side down) on a good, known flat surface (TS works pretty well here) - does the panel rock on diagonal edges, or is it stable? If it's stable go to step four. If it rocks you have wind, or twist, which must be removed before you continue.
   1. Lay the panel on a flat surface with the planed side facing up. Place a known straight stick across the panel at each end. Hunker down so you can look down the length of the panel and see both "winding sticks". What you need to do is plane the panel so that the winding sticks are parallel. Remember this ain't rocket science - it's very unlikely that you'll ever get the panel as flat as a planer will - you can get it very, very close though.
4. Now with one side flat you can mark all four edges of the panel to the thickness you desire (if you use a marking guage it will help if you darken the line with a pencil) - be sure to index the marking guage against the just flattened side of the panel.
5. Flip the panel over and secure it to your bench (Caution: if your bench isn't flat you run the risk of planing your panel to a non-flat condition mirroring the bench - if this is the case make yourself a planing board to do your planing upon - more about planing boards at the end).
6. If you have to remove a lot of material you might want to start with a scrub plane. If you don't own one of these just set your #5 for a rank cut and start planing at 45 degrees to the grain working from one end of the panel to the other. When you reach the end you want to start planing in the other direction (90 degrees to the first pass). Keep a close eye on where you are relative to your thickness marks. When you get within 3/32" of the desired thickness you switch from a scrub plane to a #5, or you set the #5 you've been using for a lighter cut and begin planing with the grain. Take the panel down about a thin 1/16" with the #5, being careful about watching the lines you marked - this should leave you a bit more than 1/32" to remove with a #4, or a #4 and a scraper.
7. Before switching to the smooth plane and scraper test your panel for flatness by laying a straight edge across it at various points and angles. If you have high spots they will most likely be in the center of the panel and you can correct them with a couple of light passes with the #5.
8. Set your smooth plane for a light cut with a tiny mouth opening and begin smoothing (again, watch your thickness marks carefully). Finish up with a scraper if you like - personally I really like the look and feel of planed wood, so whenever possible I don't go beyond the smoother.

I know this sounds like a bunch of work, but that's because you're thinking about every step when you first learn this skill. After you've done this for a while (and become confident in your hands, eyes and tools) you'll find that you do almost everything by eye without loosing accuracy.

To put this in perspective: let's say you have a panel that's 15" x 30" x 3/4" and you want to thickness and smooth it to 1/2". Assuming a little practice and sharp tools you're probably going to spend 5-10 minutes.

Planing board - this needs to be sized so that it's a bit longer and wider than the longest board you're ever going to plane. I made one of these when I was building a boat and didn't want to walk the boards into the bench when I needed to plane them. I made it in the form of a torsion box using 3/4" MDF - one layer at the top, another at the bottom and a 3" wide strip running the length of the box spaced every 2". This produced an incredibly heavy, stiff and stable planing surface that is as flat today as it was ten years ago when I first built it.

Well that's it - I hope this answered some of your questions and I hope you enjoy the process . . . rj