As hitters have many preferences when it comes to position in the batter's box, so do pitchers when it comes to position on the pitching rubber. Some will pick one spot and some will move around depending on the mound or the situation. Whatever the preference the most important factors are comfort and balance.
Some pitchers will stand with both feet lined up and some will stand with the stride leg slightly behind.
The position of the glove and pitching hand also changes from pitcher to pitcher. Some will hold the ball in the glove with the pitching arm dangling by their side. Others will have that same position but keep the ball in their hand (If any of your pitchers choose this method, make sure they use the same grip every time so they don't give away a pitch). Others will have their hand inside their glove. For the majority of pitchers the glove is held somewhere between the waist and the chest.
Most young pitchers don't have any idea how the stance can affect the final outcome of the pitch by either allowing or taking away from a smooth efficient delivery. For young pitchers, try to get them comfortable with the least amount of movement. From the stance to the windup, the least amount of movement is to have the hand in the glove gripping the ball at about chest high.
Try to get your pitchers comfortable pitching from different parts of the rubber. The main reason for this is the fact you don't have a grounds crew fixing the mound between innings or even before the game. Some mounds will be in such poor condition that fixing them at game time will be impossible. A pitcher will need to be able to pitch comfortably from different areas of the rubber to utilize the best possible landing zone for the stride foot on a poor mound.
If the pitcher starts with one leg back this can be a weight shift back.
A common problem is taking too large a step back. Again, the more movement the more difficult it will be for the pitcher to be balanced during the windup. Watch most major league pitchers and you will see they take a very small or no step back to start their windup.
As the weight is transferred to the back leg, the front foot will be turned and placed parallel to, in contact with, and in front of the rubber. The feet are now in position to begin the kick.
It is important to realize that when we talk about balance for the pitcher's windup, it doesn't mean that the pitcher should be straight up and down and be able to hold that position at the top of the leg kick. Often that's what is thought of when we talk about a pitcher and balance. Rather think of balance in the pitching motion as side to side rather than up and down. We want our pitchers to be balanced from side to side as they go through their entire pitching motion. If not it will be impossible to develop a consistent pitching motion that results in good control.
At the same time the pitcher takes the step back, the arms will also move. Pitchers will either swing both arms over the top of their heads as they step back and begin pivoting their front foot (not very common), or (more likely) they will keep their hands in front of their chest and move directly into the top of the kick from that position.
Once the front foot has pivoted and the hands have reached the top. The pitcher will shift his weight onto the pivoted foot and pull the back leg forward and up, swiveling as he does this until the his thigh is parallel to the ground or a little higher. His body should be sideways to the plate. Make sure your planted leg is not locked at the knee, it should be slightly flexed.
The critical element is balance. That doesn't mean that the pitcher should be able to hold this position. In fact, the pitcher should be moving forward as he reaches the top of the kick and should have some forward lean to his body (towards home plate) to help get moving in that direction. Many young pitchers are taught (and I used to teach as well) to get to the balance point where you can hold that position. While this is supposed to show good balance it really only helps if the goal is to stop there. Since our goal is to pitch the ball, a slight forward lean will actually help the young pitcher keep their entire motion in balance because they won't have the opportunity to go too far back at the top of their kick and get their balance going in the wrong direction.
The pitching motion is a combination of many movements that need to be executed exactly the same way with every pitch. Without balance at this point consistency in the pitching motion is impossible and without consistency in the motion, there is no chance of having good control.