I have a blade putter and a mallet putter. Recently I picked up a 43inch belly putter and LOVE it.
I let the butt end of the putter ride against the top half of my belly button and grip it just like a normal putting grip - reverse overlap. The putter moves with horizontal hinging. I have found that ball position is the most critical element with this putter. And dead-center in the stance seems to work best. The other key element was finding the best place for the butt of the club.