Andy Gorman is a PGA Professional and a specialist putting coach. In this putting drill he talks about what putting fundamentals you need to practice irrespective of your handicap.
The key points that Andy makes in regards to posture when putting are:
1- Stand upright and not crouched over the ball
2- Keep a straight spine angle
3- Relax the arms and let them hang loose from the shoulders
Andy says getting into a poor posture when over the ball will result in you struggling to practice for more than a couple of hours, when really you don’t need to.
Keeping your spine angle straight and allowing your arms to hang freely from the shoulders will only help you out on the putting greens.