From a standing position, reach out and step with one leg. The step should be the length of about 1 and 1/2 to 2 steps. From here you lunge down trying to keep a straight line down your back to your trailing knee until you reach 90 degrees with your leading knee. The goal is to not let your leading knee track passed your toes. This is controlled by your step distance and maintaining that straight line. As you come out of the lunge, you want to push through your heels and not your toes. Try to explode out of your heel back to the upright standing position. Now repeat making your other leg the leading leg this time.
The downward motion of the lunge primarily uses your quad where the upward movement is more of a hamstring workout. Some hip flexor and core are also activated throughout this movement.