Stand in a stance wide enough to fit a kettlebell between your feet. Maintaining a neutral spine pull your hips back and lean over to the bar. You should find yourself with your shoulders over 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 to a natural position. Be sure to pull the bar in tight to your body while standing keeping the bar against your legs.
Lower the bar by leaning forward and pulling your hips back, maintaing neutral spine and keeping the bar tight against your legs is essential. Ideally you will finish lowering the bar in the exact position you began the movement in. If not stand up and reset your position.
If you notice your knees keep shooting forward before the kettlebell has passed them then you are not pulling your hips back or leaning forward.