The basic how-to: Hold the steel in your nondominant hand and rest the tip firmly on a folded dish towel. The steel should be perpendicular to the countertop or table. Then take the knife (which should not be any longer than the steel) in your dominant hand, placing it behind and perpendicular to the steel, with the back of the blade (the part nearest the handle) against the steel. Pull the blade toward you while gliding it down the steel. Do this 10 times on one side; then bring the blade to the front of the steel and do the blade's other side 10 times.
Common mistake: Not sharpening knives often enough. Keep the steel handy and use it whenever you notice your knives are slipping on food instead of slicing through. This can be as often as once a week if you cook a lot.