To begin place the bar on the back of your shoulders just like a squat. Put one foot well out in front of the other in a staggered position. Take the other foot and put it up on a bench or box no higher than 18 inches or so. Maintain a tall and tight torso throughout the exercise and keep your front heel down into the ground. let yourself drop straight down to the floor stopping just before the back leg hits. The brunt of the work is on the front leg while the back leg maintains balance.
If you tend to be extra tight in the back you might want to use a lower bench or box for the back leg. This exercise can be done with the barbell in front of you or with dumbbells being held at your sides. I like to cycle this in for squats and tend to heavily favor it on leg days.