Try to take your hands out of the putt. By that I mean no independent hand/wrist action.

Let your arm arms hang comfortably no tension don't hole the handle tight.

It's best not to use the conventional grip that you'd use for the other clubs where the handle (grip) is mostly, or should be in the fingers, you don't want this with a putter as your trying to stop hand action not allow it to happen.

With a putt (assuming you're a RH golfer you want the putter handle to lie in your left palm vertically from the base of thumb (the fleshy part of your palm) down in line with your fingers  that are pointing down to the ground, then close your fingers around the handle left thumb goes vertically down the front on the putter handle, your right hand fits around, right little finger overlaps left forefinger, handle is more in the palm of you right hand too. You want this to stop your wrists from breaking down and becoming involved in a hit at impact.

The motion, backswing to throughswing, and distance is controlled by the shoulders in a rocking motion, the negative shape made by the arms between the hands and the shoulders needs to remain the same throughout the stroke no breaking down of the arms at the elbows.

The pace of the backswing & throughswing is better if it's the same speed back & through (count 1 & 2 as you move the putter back 7 through it will help regulate the tempo of the stroke), no hit or quick acceleration at impact.

The lenght of the stroke back & through controls pace and distance the ball rolls. It's a matter of practice on the greens you normally play it to find the lengths of the strokes to control the different distances.

Goes without saying you need a square face through impact, and therefore a square face to the target line of the putt at address. It helps to pick a spot out just in front of the ball on your line to the hole and imagine rolling the ball over that spot and into the hole.

You also need solid legs , no other movement other than shoulders & arms, you head needs to be stock still, don't lift up. 

Trick to help this. Imagine there's an imaginery coin under the ball and you don't know whether it's heads or tails looking up at you, make the stroke but keep looking at where the coin would be so if it was there once he ball has gone you would see whether it was heads or tails!  Good luck.

Cheers.