When I teach kids, or I'm helping an adult who wants to learn to pitch the first thing I tell them is prepare to be humbled, because there is no more humbling experience in the world than learning to pitch. You will bounce it, throw it off the backstop and everywhere in between. Then when you start to get comfortable you'll get ahead of batters and throw four straight balls to walk them, or hit a guy on an 0-2 pitch. All of this will be done as guys on the opposing team trash talk or ***** about you not having enough control (especially if you throw hard).
If you're prepared for that, and think you can get through it, then you'll be a great pitcher and it mentally prepares you for the toughest moments you'll face in a league playoff or a tournament.
As for actualy mechanics, my advice:
1. Find the same release point all the time, and master that first.
2. Make that arm act like a pendulum, and get a good swing. Like your hands in hitting they should accelerate through the downswing, and don't try to over throw/swing with your shoulders.
3. Don't even notice the batter. Whether I'm throwing BP to my team, or pitching in a real live game I focus on a catcher's glove, a screen, or a pole on the fence of the backstop and try to hit that.
4. Use the legs, just step and throw, if the lower half is dead, you'll have less consistency finding a release point, and it may ultimately lead to injury throwing all arm, and shoulders, or awkward elbow contortions.
5. For the modified guys I also encourage throwing a little windmill, learning the full arm swing, even if its just lightly makes learning the halfswing, going backwards first feel a little less awkward.