We always look to see how they throw first as to throwing efficiently or on a straight line or linear would be required. Getting them to understand they lose power by stepping in or out helps also. See it, feel it and fix it. I use a crutch and place the padded side on the upper thigh of the back leg. As they turn the hip IF the front foot is not at about 40 to 45 degrees they will not allow the weight to transfer forward very well. They will feel they can not transfer the weight forward very efficiently. Demonstrate by having them step open and closed to feel the difference and they usually pick straight ahead.
I would also look at the distance they measure off from the plate. You want a good two eyed look. Sometimes they step that way because they can not see the ball out of the pitchers hand. We use a bat length from the outside edge of home plate. We have found they can not look forward well enough IF they do not open their stance by 2 to 4 inches. Tape a pencil or straw and attach it to their helmet in the center on the bill of the helmet. First keep both feet even. Ask them to look forward at the pitchers window of release. Then ask them if there is any discomfort in their neck or shoulders. Look at the direction of the pencil and most likely it will be right of second base if they are a right handed hitter. Now ask them to step back with their lead foot about 2 inches and then 4 inches and ask them how it feels. The pencil should be pointed forward. Use whatever you choose to start their load and separation as to the pitcher's motion as there is a lot of style that some choose to interject in how they teach it. Same thing applies to stride width and stance as many use different styles.
We use a penny, nickel and dime. Once you get your stance place the penny at the toe of the back foot. When they can get the two eyed look comfortably place the nickel at the lead foot toe which is offset. We stride to a linear or straight line about the length of their bat and place the dime at they replanted lead foot. The stride is usually the length of their bat and we place the dime at the lead foot toe and in line with the penny. This is the same position to throw from, hit from and field from. Ask them IF they sway backwards to field a ground ball and they will not be able to lower their body without making their legs look like the shape of a pyramid.
They can do this inside the house without even using a bat and have them do it with their eyes closed. IF they feel it they will feel toe to toe (big toe) pressure by shifting the weight knee to knee and not swaying. About 7 to 12 minutes a day and they master it pretty quickly. When they stride ask them how it FEELS and then open the eyes and see where they actually are. Measurable, observable and we tested to see if they could feel it.
Howard