Starting Position: Stand with your feet together. Pull your shoulder blades toward your hips. Engage your abdominal/core muscles ("brace") to stabilize your spine.
In preparation to step forward, slowly lift one foot off the floor, find your balance on the standing leg. Try not to move the standing foot and maintain balance without wobbling. Pause. Hold this position briefly before stepping forward. The raised foot should land on the heel first. Slowly shift your body weight into the lead foot, placing it firmly on the floor. As you shift your body weight to the lead foot/leg, avoid the tendency to tilt or sway the upper body and try not to move the forward foot.
As you step forward into the lunge, focus on a downward movement of your hips toward the floor. Avoid driving your hips forward. This will help control the forward movement of your shinbone over your foot. Continue lowering your body to a comfortable position or until your front thigh becomes parallel with the floor and your shinbone is in a slight forward lean. During the movement, slightly bend forward at your hips and squeeze the glutes of the back leg, this will help increase the stretch in the back hip flexor muscles. Keep the back straight.
Hold this stretch position for 15-30 seconds. Repeat for a total of 2-4 repetitions. Try to move into the stretch a little deeper with each repetition, but be sure to keep your body in alignment. Complete all repetitions on one side before alternating to the other leg. Firmly push off with the front leg, activating both your thighs and butt muscles to return to your upright, starting position.
At the bottom of the movement: contracting your abdominals and the glutes of the back leg while simultaneously lifting your chest up to the ceiling will help increase the stretch to the back leg.