Sitting Position: The 90º Rule
It is important to play with good posture as to not develop long term back and shoulder problems while playing the guitar. The ideal playing position follows the “90 degree Rule” which is an easy way to remember the correct posture. All of your joints in your lower body (ankles, knees, and hips) should be at 90º angles when seated.
In order to create this position, your left foot should be flat on the floor, and your right foot should be at the same angle regardless of wether you use a footstool, or if you use a guitar strap or support system that will keep both feet flat on the ground.
Your chair should be at a hight where you can keep this 90º position in your feet, knees and hips. Using a piano bench or a chair with an adjustable height will be ideal for finding the proper playing height.
It is also extremely important to not sit back in the chair, but to sit forward on the chair to create a straight back and position the guitar correctly across the body. Notice the “triangle” created between the body of the guitar and the torso of the player. This will ensure the proper playing position.