Hold the barbell on the shoulders, similar to a back squat.

Standing with feet hip-width apart, dig the big toe in each foot into the ground. 

Tighten the core as much as possible, imagining that someone is about to punch you in the stomach.

Allow the hips to slide back and the upper body to lean forward while keeping the barbell close enough to the body that it is sliding down your thighs. As you are descending, drive the knees outward, as this will allow you to better employ the glutes. Feel the weight being absorbed by the hips. Descend until the torso is close to parallel with the ground, feeling for a good stretch taking place in the hamstrings. If you are not very flexible, go down until you feel a good stretch, instead of trying to get the torso parallel with the ground. If you are very flexible, only go down to a parallel torso.

Rise up to the starting position, focusing on the hamstrings contracting and doing all of the work, NOT the lower back. Squeeze the glutes at the top by performing a posterior pelvic tilt (pointing the tailbone forward).