This is a common and simple exercise.
1. Stand erect facing forward with good posture, i.e., eyes forward, chest up, etc.
2. Step forward into a long stance, stretching the back leg, while squatting down with the leading leg. The length of your step should be such that your knee does not protrude past your toes when you squat.
3. The depth of your squat should be such that the squatting legs thigh is horizontal to the ground, while the trailing leg had the knee about 3-4 inches off the ground.
4. Standing up, step forward so that the trailing leg is now the leading leg. Once you have done squats on both legs you have completed one repetition.
5. Generally, people will walk around a gym, track, etc. until they are pretty burned out. You can also do a specific number of repetitions per set or a set distance for a set.