Watch the Elbow of Left Arm online guitar lesson by Bruce Arnold from Guitar Physiology Survival Guide
The elbow moves your hand to move around the neck. It should remain about 2 to 3 inches from the side of your body, otherwise it will cause shoulder problems. You can place a thin pillow around your waist to keep your elbow at the correct place.
Do that for a week or two and your elbow will automatically go to the right place when you are playing. Sitting while practicing can lead to a bad habit of keeping your elbow too close to your body. This in turn forces a series of awkward angles onto your whole arm and is a common cause of repetitive stress injuries.