Instructions
Stand up tall with your hands by your sides or on your hips. Position your feet so that they are a fist distance apart. Take a large step forward with one leg and position your back foot so that your heel is up. Lower your body down by bending your knees. Keep lowering until your back knee is touching or almost touching the floor. At the bottom of the lunge your legs should both be close to a 90 degree angle. Return by forcibly extending the knee and hip of the front leg and stepping back to the original standing position. Repeat on the other leg and keep alternating legs until you've completed the desired number of repetitions.
Tips
Your torso should be kept upright throughout the whole movement. Hip mobility may prevent some people from lowering their back knee to be close to the floor. In this case, lower down as far as possible as not to cause any irritation. This exercise can also be performed in a walking style or by alternating with a backward step (as opposed to a front step demonstrated in this video).