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A simple way of practicing your lunge technique and strengthen the muscles supporting the lunge is to perform it in isolation.
Stand with both feet shoulder width apart, step out on one leg, lunge and hold briefly before pushing back. Do the same on your other leg.
Depending on your fitness, perform 10-20 alternating leg lunges before resting. Repeat this 3-4 times. You can also practice lunging at different angles and heights.
If you are able to do this in front of a mirror then check that you knee remains in line with the foot and does not wobble.