Str8Putt should never make you feel uncomfortable in your setup. This is imperative, make sure you are settled into your stance the way you normally do.
Set the parallel arms about an inch below your putter grip, probably close to the sticker if you have one.
Place your putter shaft at the orange dot on the arms, third dot in.
Align your shoulders parallel to the arms. Work hard to make this happen as your best stroke will be a result.
Once you have determined your best and most comfortable position (not to say you can't change it later) place a coin next to the ball so that you easily identify where the next ball goes.
Start with the arms at about an 1 !/4" apart to get a feel for it. As you progress without hitting the arms you can move the arms closer together. Or be a hero and start at 3/4" and see where you go from there.
DO NOT attempt to line yourself up at hole while using this. Set tees at 4',6' and 8' to work on speed control and making a good stroke. Your reward is not hitting the bars and making a good stroke. Putts are made on the course when they matter!
You will find many uses but one that is really important is eye position. Open the arms to about an 1 1/4" and set the ball directly under the orange dot. Now take your stance like you would normally and look at the ball through the arms. This will also help you stay steady over the ball as the arms will appear to "move" if you do during the stroke.
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