The keys to putting left hand down are get all fingers of your right hand on the grip
Use a light grip around 3 out of ten if ten is max tension. Its about the same tension you would grip a tube of tooth paste.
Have the ball a little in front of middle so you don't get the ball bouncing, you just get a pure roll. (If it bounces just move the ball a little further forward in your stance until you get roll of the putter face.)
Have the back of the hand and fore arms almost straight on both arms. (To achieve this you will have your thumbs crossing the grip. This will make it very hard to get wristy in the putting stroke.
Finally have a stroke that is 1/3rd back with a 2/3 follow through.This will stop you decelerating on impact.
Have a look at Hans Olofsson in the world putt putt championships for a accurate but unorthodox left hand down style. Check the follow through.