Rip-T Drill
The absolute best way to teach pitching mechanics to a young pitcher is to break it down into multiple steps that they can master, then put everything together at the end.
The Rip-T drill is the first step in not just pitching correctly, but throwing a ball correctly.
The Drill:
Starting in the "stretch" position (the windup is 2 extra steps that can be taught later), have the player start with their hand in their glove in a pitching position. They should simply circle both hands so that their glove hand is pointing to their target and their throwing arm is bent at the elbow, elbow slightly higher than their back shoulder, with the ball pointed backwards away from their target.
Points of Emphasis:
1. Make sure the ball is pointed backwards (away from the target). If not, the player will end up throwing like it is a pie (pushing it).
2. Keep your eye on their grip to make sure they are using either a 4-seam (preferred) or a 2-seam.
3. Make sure they are circling their arms, and not directly pulling them out of the glove and up in the air.
4. If available (if not, tell them to practice at home) - find a reflective surface (window/mirror/turned-off tv) and have them watch themselves.