The first and most important step in doing a proper squat is checking your posture and stance. You should be standing with your chest up. Keeping your chest up will help make it impossible for your lower back to round and you will be able to tighten your upper back easily. You should always be looking forward, never down or up. Watch where you place the bar, it should be low on the back of the shoulders, so that it is sitting on the bone at the top of the shoulder blades. Your grip should be narrow so that it is easier for you to tighten your upper back. Also your thumbs should be next to your fingers on top of the bar.