Set up a barbell with full sized plates. Stand in a hip-width stance. Maintaining a neutral spine pull your hips back until you can reach the bar. You will have to allow the knees to bend some so that your hips can get all the way back and to allow the last few inches of reach toward the bar. You should find yourself with your shoulders slightly in front of the bar and hips at a height between your shoulders and knees. Firmly grab the bar just outside of your legs keeping your shoulders down and back.
Begin to lift the bar by driving your heels into the ground and standing up fully. Be sure to pull the bar in close to your body while standing, keeping the bar against your legs.
Lower the bar by pulling your hips back, maintaining neutral spine and keeping the bar tight against your legs. Ideally you will finish by placing the bar in the exact same position you began the movement in. If the bar returns to the ground further in front of you than you started, you pushed either your knees or your shoulders too far forward
If you notice your knees shooting forward before the bar has passed them, you are not pulling your hips back enough. You must allow your upper body to tilt or you will not be able to get your hips far enough back. Your back must remain neutral, it will not remain vertical.