There are many different ways of holding the putter, but the most common is the reverse overlap grip. This places the putter in the palms of the hands and takes the wrists out of the equation, allowing you to keep the putter square to the target.
With the putter placed in your palms, wrap the fingers around the grip so that both thumbs sit on top of the grip. Overlap the left index finger over the index finger of the right hand to join the hands. Grip the putter with light pressure for maximum feel and control.