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.
I have accidently built one and I'm anxious to try it out. I say accidently because I what I was actually building was my version of the hAMMY putter. The guy who invented it sent it to me to try out, but I didn't care for the anser style head. I built mine with a mallet head I had bought from a component shop a long time ago and never used. But while messing around with it on the carpet, I realized the length was just right for a belly putter. I have never tried belly putting before but I liked the way I was stroking it on saxony plush. I need to head down to Edwin Watts and pick up a long grip for it and try it on bermuda!