When you have a loop in your swing (most amateurs swing back inside and then loop the club to the outside) you must make a compensation to get back to the ball. That hurts your consistency.
To check and improve your swing path, place an alignment stick or club on the ground behind the ball and in line with the target, choke up on a long iron to about six inches from the head and take your stance. At address, make sure the clubface is square to the target (1). Then make a slow backswing and stop when the club is parallel to the ground. The shaft should be parallel to the stick__(2)__. Finally, at the top, the extended grip end should point at the stick (3). If it points inside or outside, you'll have to loop the club to get it back on path.
When I'm teaching putting outdoors, I'll snap a chalk line on the green to create a straight-line guide. Indoors, you can use a carpenter's laser to do the same.
First, draw a stripe completely around the middle of a ball. Then set the laser behind the putterhead so it shoots a line down the carpet (right). Place the ball so the stripe is right on the laser line, then practice rolling the ball so it goes down the line and the stripe doesn't wobble.
If the ball strays from the line or the stripe wobbles, it means the putterface wasn't square to your path at impact, or you cut across the ball.
The average 10-handicapper makes about 3½ driving errors per round. (A driving error is defined as a shot that results in a penalty or prevents you from hitting your normal next shot.) The average 5-handicapper, who regularly shoots in the 70s, averages fewer than 2½driving errors.
—Peter Sanders / shotbyshot.com game analysis