The Single Leg Dead Lift starts with a stance that is narrower than for the Barbell Dead Lift. The barbell will be very close to the same positioning as a Barbell Dead lift, close to the shins. Maintain a neutral spine by bracing your butt and abs then pull your hips back. Hinge through your hips to bring you down 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 barbell. Begin to lift the barbell by driving your heel into the ground and standing up to a natural position. Be sure to pull the barbell in tight to your body while standing keeping the bar against your legs.
Lower the barbell by leaning forward and pulling your hips back, maintaining a neutral spine and keeping the barbell close is essential. Ideally you will finish lowering the barbell in exactly the position you began in. If not stand up and reset.
If you notice your knees keep shooting forward or in before the barbell has passed them then you are not pulling your hips back or leaning forward.