What to do and what not to do (and things that do not matter).

The collective knowledge of great pitchers is far greater than anything I claim to know. Pitchers are always looking for an edge in technique that will give them a competitive advantage of some sort. Observing what they do as they execute a pitch, as opposed to listening to what they say they do, is very beneficial in determining what to teach. If all the great pitchers do some part of the dellivery the same way, that is something you should do. If there is something that none of the pitchers do, that's something you shouldn't do. If there is somethng that some pitchers do one way and others do differently, it probably doesn't matter that much.

The cdritical part of the delivery happens so fast that observation of live pitching will not tell you what the pitchers are doing unless you have a very trained eye and look for specific things like college scouts do. Analyzing slow motion high defenition video shows the similarities of the great pitchers and the different approaches used by less than great pitchers.

What follows are my observations based on many hours of video study of both great and not so great pitchers.

Before starting the pitching motion:
Do: Take the throw from the catcher in front of the pitching plate, turn and walk behind the plate, think about any fixes you need to make for the next pitch, breathe and get on the plate to take a sign.
Don't: Think about the previous pitch, the result, the umpire's call or anything that annoys you once you have turned back to walk up to the pitching plate.
Doesn't Matter: Any pre pitch action that makes you get focused such as touching a rosin bag, finding a spot to focus on, looking at your teammates, saying a mantra. Whatever method you use to narrow your focus and clear your mind works.

Before the first quarter of the arm circle:
Do: Move your center of gravity as far back as is comfortable
Don't: Don't do anything that makes it hard to get into proper position for the remaining part of the pitching motion. For example many pitchers will start by swinging their throwing arm behind them. This turns the shoulders into a somewhat open position. The shoulders must recover to squarely face the batter during the push and stride. Great pitchers do this, beginners often keep their shoulders open during the stride which reduces power.
Doesn't Matter: Whether or not you swing your arm behind you, whether any arm swing is with one or both arms.

During the first quarter of the circle:
Do: Snap your hips forward to be directly under the shoulders, push off while squared up to home plate,
Don't: Turn your stride foot sideways before pushing off, have the stride foot knee cross the power line
Doesn't Matter: Whether your hand is in front of, om the side of or behind the ball

During the second quarter of the circle:
Do: Turn your hips and shoulders to be completely sideways by toe touch, have the hand on top of the ball, bring the ball up in front of the face
Don't: Let the scolders be closer to home plate than the hips,twist your forearm or wrist. don't bring the ball up on the power line
Doesn't Matter: If you bend slightly at the waist or stand straight up.

During the third quarter of the circle:
Do: Arrive at toe touch with the ball facing third base (RHP), land the stride foot near the power line at a 45 degree angle, toe first, knee flexed but firm
Don't: Cock the wrist, turn the ball to face second base, straighten the elbow
Doesn't matter: Everything matters which is why all great pitchers do everything in this phase the same as each other (see note below)

During the fourth quarter of the circle:
Do: Arrive at pre-release (upper arm straight up and down) with 30 to 45 degrees of flex in the elbow and the ball still facing third base, do be straight up (on the leaning forward or backwards plane) at release, allow the elbow snap to rotate the hand towards home, move the upper arm faster in the 4th quarter than the third,have resistance in the front leg so that the energy used in moving the body forward can be transferred to the arm whip.
Don't: Lose any elbow flex until pre-release (keep it the same or increase it), don't attempt to snap the wrist at release
Doesn't matter: Everything matters which is why all great pitchers do everything in this phase the same as each other (see note below)

During the follow through:
Allow the elbow to release, arrive at a position with the elbow fully straightened and the back of the hand facing the sky or towards third base(RHP), remain very loose in the wrist and elbow joints
Don't: Stop the elbow at the hip
Doesn't matter: After the arm reaches the fully straight position above, the hand can turn back towards second with the elbow bent, go the the glove hand shoulder, or remain pointing at the batter, whether you turn your hips and shoulders to face the batter or leave them open, whether your balanced finish is on one leg (for example the figure four) or two.

Note: When everything matters is used it doesn't mean everything has to be done to exactly the same degree. For example, it matters if you arrive at pre-release with some flex in the elbow or not. That is critically important. The general range of practice for great pitchers is 30 - 45 degrees of flex, and where in there, or even somewhat outside of there, doesn't matter. So great pitchers doing everything the same is more of a bigger picture concept rather than a micrometer measurement. They all arrive at pre-release with flex but some have more and others less. That's the same in my book.