If you are right handed player, then grasp the belly putter with your left hand, in a way that your thumb should be on top of the shaft and you hold the putter with your fingers. You left hand should be extended out, and your knuckles should be facing away from your body.
Now pull the putter towards your body in a way that the shaft where you placed your thumb should touch your belly button. Bend a little and you should be leaning over the putter and your eyes should be exactly over the ball, if you look down.
Make sure that the belly putter is positioned vertically with your belly button to the putting surface. Here, you need to customise the length of the belly putter, if it is not making a perfect angle. If it is perfect, then your left arm elbow should face out forward and going away from the body, while the arm making almost 90 degree angle.
Now place your right hand just below your left hand on the shaft. Make sure you do not hold the shaft with your right hand palm. In fact, you should hold it with your fingers. Your right hand will now look like a claw, while holding the putter.
You will now see that your right hand is around 10 inches below your left hand and will remain straight towards the club.
Now pull the belly putter back using your right hand and swing through the golf ball. You will see that the top of the putter will not move, as it rests at your belly button. You will also witness that you will not have to apply any wrist movement while making putt using belly putter.