Marshall I used an online ballistics calculator (selecting 300 meter zero) as a guide to setting our SKS sights. I started out with my rear sight on the "battle site" setting Marcus speaks of. Make a note of where the bullet crosses your line of sight at 25 meters in the data generated by the calculator, for example in the calculator I used it was approx 2 inches above the axis of the bore. I set up a target at 25 meters and adjusted our sights to strike two inches above the bullseye/point of aim and then shot at longer known distances (100 and 200 meters) to make fine adjustments.
Marcus' advice is critical at this stage, also if possible try to do this on a calm day when you are not in a rush. And remember these online ballistic calculators should ONLY be used as a guide, their data is generated in a controlled environment with a test barrel! For my wife and I this worked very well and we were pleased with the results. As long as you keep in mind these little critters are not tack-drivers!
I don't know if this is true but I have read the Russians gave their recruits a rule-of-thumb when shooting at various ranges with the SKS rear sight set at the battle-sight setting. At long distances aim for the head, at short distances aim at the belt buckle...makes sense considering the rise and fall of the projectile!