Plumb bobbing is a technique some golfers use to line up putts by using a putter as a plumb bob to create a vertical line they can use to compare the slope of the green. Plumb bobbing is a way to get a general feel for the overall movement of the green, not a way to line up putts. Unfortunately, most resources on plumb bobbing demonstrate a faulty understanding, instructing golfers to line up putts based on data that are physically anachronistic.
Squat or kneel 10 feet behind your ball so that the ball is directly in between you and the hole. For longer putts, you may want to move even farther behind the ball, perhaps as much as 20 feet.
Hold the putter between thumb and forefinger and allow it to hang freely. Point the toe of the putter directly at the hole, away from yourself, to make sure the shaft appears to hang vertically from your perspective.
Adjust the putter and the position of your head as necessary until the silhouette of the putter occludes both the ball and the hole. No matter the slope of the green, this is always possible. Plumb bobbing instructions that claim that you can identify the break depending on which side of the hole the shaft appears to hang on are flawed, lacking a basic understanding of geometry.
Observe the slope of the green around the shaft. If the green is higher on the left, the putt will break right; if the green is higher on the right, the putt will break left.