1. Remove the bindings, measure the length of the board, and put a sticky note in the center (across the waist line).
2. Your stance width should be the width of your shoulders, or about 1/3 of your height (20" for the average sized male). Place your bindings on the board so they are the proper distance from each other AND the stance location (the half waypoint between the two bindings) is a few inches behind the waist line. This shifts your weight back a bit, giving the nose a bit of lift for riding over powder. This step is mostly trial and error. It also helps if they actually line up with the binding holes :) (If you're especially tall, or if following this step doesn't yield a comfortable stance width, see the comment below)
3. Set your stance angle. I use +20° in the front and 0° in the back.
4. Center the boot across the snowboard by sliding the binding back and forth (notice there's a row of screw holes to choose from). This is done after the stance angle is set because the angle affects the effective length of the boot (i.e. the bigger the angle, the shorter your boot seems). Too far in either direction and you won't be able to turn. Even worse, too far toeside and you'll drag your toe when you turn toeside. Also, ever notice how you drift one way or another when your back foot isn't strapped in? This might be why.
5. Once everything is lined up, screw in your bindings. Tight. No, tighter. They should be tight enough that if you use all your strength, you can't tighen them another iota. Binding screws are notorious for coming loose at the worst time. Tighten them every few trips up the mountain.
6. Set your forward lean, by loosening and adjusting that mysterious tab at the base of your highback. I've heard that 10° is average, but honestly how can you tell what the angle is when there isn't a reference point at 0°? So this is really something you'll have to tweak a bit, then strap in, and tweak some more. A healthy amount of lean will force your knees to bend into a proper riding stance. Too much will put excess strain on your quads, tiring you out too quick and making it hard to turn.
That's about it. Expect this whole thing to take 30 minutes or so. Have fun up there!
If you found this guide helpful, please consider making a small donation to Outdoors For All, a charity that helps disabled people ski and snowboard. Thanks!
abc-of-snowboarding (this one has some advanced tips, such as adjusting highback rotation)