You’ll need to find a playing position where you can maintain the relaxed, grounded sensation you achieved with the progressive relaxation and focused awareness exercises. The first step is to get a solid stance that will allow your upper body to relax. This stance is called ‘horse stance’ because it resembles the position you take when riding a horse. The aim is to create a posture in which your legs take the weight of your upper body, allowing it to relax:

This is the basic stance in tai chi practice and and also work well when you're playing your guitar. It typically takes some time to become comfortable with, short hamstrings tend to fatigue and lock the knees up every now and then. Hamstring stretches will help as will keeping the hip muscles relaxed and maintaining tone in the core hip flexor and abdominal muscles.