These instructions assume a right-handed player. Just switch the hands if you're left-handed.
1. Sit forward in your chair and sit up straight. Resist the temptation to slouch, or lean your left arm on on your leg.
2. Let your arms hang by your sides then bend your left arm at the elbow until it is parallel to the floor. Turn your palm up, and this is essentially where your hand should be positioned when you play.
3. Place the guitar on your right leg and angle it so that the neck is aimed at your lifted left arm.
4. Keeping your left arm in position (don't move it at this time to meet the guitar), move the guitar forward or back on your hip until it begins to feel balanced and the neck is roughly in the correct position over your left hand.
5. Use your right arm to gently hug the guitar into your rib cage and fine-tune the position. Now you can adjust the exact location of your left hand on the neck, the angle of the guitar, its placement on your hip, and the height of the headstock. Curl your left-hand fingers around the neck and you're ready to go!
For the beginner, these steps could help avoid many hours of frustration and wondering why it's so hard to play certain things. For the intermediate or advanced player, especially if you are experiencing any discomfort while playing, evaluating the position of the guitar could alleviate many difficulties.