G'Day Tim,
A good question and a common problem. This answer is longish and may take a couple of re-reads, but bear with me, it works.
Put your hand in front of you now so you're looking at your palm. Now hold your hand like you were throwing a cricket ball. What fingers are you wrapping around the imaginary ball? Thumb, index finger and middle finger, right? According to then Team USA head coach Fred Borden way back in the 90's, these "pincer" fingers are 40% stronger than the next two. (No surprise when you think about it.)
I sometimes imagine a "virtual finger" between these two fingers and that I drive the ball forward by using this virtual finger behind the ball. What this does is allows you to feel the balance point in the strongest part of your hand. That's the hard bit dealt with.
The next part involves your upper arm. Most early-turning or over-turning problems result from the bowler rotating their arm instead of just their forearm & wrist.
Put your arm in front of you like before and place a couple of fingers on your forearm just below the elbow, so you can feel if your upper arm and elbow move. Now turn your hand and wrist. (Your elbow must not budge.) Try it a few times until your elbow is solid like a rock and your forearm/wrist moves fluidly. Notice that if your elbow stays under, your wrist stops over turning. You only over-turn when you "chicken-wing" with your elbow.
I hope this helps. Feel free to ask more via PM if you need to.
Good Luck
Jason Doust