The stretch position is used whenever there are runners on base. You start with your back foot against and parallel with the rubber. Your feet should be approximately shoulder width apart with most of your weight toward your back leg. You move into the set position by bring your hands together in the center of your body. Some right handed pitchers will have a slightly open stance when they come set, this allows them a better view of the runner at first base. While the view may be better, it will open up your left shoulder and make for a longer delivery and could cause poor rotation. Strive to be in a good position to pitch from and work on your ability to see the runner by turning your head.
From the set position, lift your leg to its highest position and make sure to have your back knee flexed, not locked. If you're a right handed pitcher you will want to develop a slide step to help out your catcher in trying to throw out a runner trying to steal. The slide step will require that you don't lift your leg as high before striding. The slide step is difficult to master and younger players shouldn't worry about it. For young pitchers just learning the stretch position, you should try and help them get to the same throwing position at the top of the leg lift that they are in when they windup. This will help them become consistent whether throwing from the stretch or the windup.
Left handed pitchers are at an advantage with a runner on first in that a normal leg kick will allow them to go to the plate or to first base. A slide step isn't necessary with a runner on first base.
It's essential to remember that pitching from the stretch doesn't require a different pitching motion. Once you get to the highest point of your leg kick everything should be exactly the same as when you pitch from the windup.