If you setup with your putter on the ground, you have to lift it before starting your stroke.  Lifting the club often causes the stroke to pull to the left during impact with the ball.  Instead, just brush the grass at address so that it is already level and does not need to be lifted.  In this manner your club goes only back and not up giving you a smoother and more controlled stroke.

Here’s a tip to help you control the distance of your putt. Set up about 6-8 feet away from the pin and use only your right hand to swing the club. Keeping your wrist as stationary as possible. The more you cock and uncock your wrist the less control you have over your speed. Keeping your wrist stationary throughout the stroke will keep your distance under control.

This is a great 30 second commercial for the Ping Iseries Golf Putter.