The plumbing technique, also called plumb-bobbing, relies on using a dangling putter to provide a vertical counterpoint against which the horizontal slope of the green can be evaluated. Unfortunately, the practice is widely misunderstood and misused. Many golfers believe that the plumb line provided by the putter will not line up with the ball and the hole, and that the difference indicates the break of the putt. This belief is nonsensical: The alignment of ball and hole with the vertical is purely perspective. If they don't line up from one angle, all you need to do is move over to a new spot. The plumbing technique can still be useful, however, as a means of orienting the green's slope with the vertical.
Line yourself up behind your ball on the green so that the ball is directly between you and the hole.
Hold up your putter by the handle, either directly on the grip or directly below the grip. Point the toe of the putter directly away from yourself or directly toward yourself to ensure that the line of the shaft appears vertical. If you point the toe sideways, the shaft will appear slanted.
Align yourself so that the shaft appears to cover both ball and hole. Often, it is helpful to close one eye to gain a clearer perspective.
Observe the contours of the green as they cross the putter shaft between the ball and the hole. If the contour crosses at a right angle, the green is flat, from a left-right slope. Remember that the plumb-line technique tells you nothing about the uphill or downhill slope on a putt.