Learning how to lunge a horse is not difficult, provided the horse already knows how to lunge.
Snap a lunge line onto your horses halter then step back towards the horse's hindquarters and give a verbal clue for the horse to move (usually a cluck or smooch) Your horse should begin to move out in a circle around you. To maintain forward motion, your body should remain behind the horses eye as they move in a circle. How to lunge a horse is a skill best taught how by a professional trainer- at least your first few times.
once you learn how to lunge a horse it's not a practice that should be overused. Horses easily grow bored when lunging so unless you have a specific goal of teaching respect or improving a gait while lunging, it's best not to lunge very often. Some people lunge a horse before riding to tire the horse out before mounting. This works temporarily to make the more more manageable, but the more a rider lunges to tire the horse, the more conditioned a horse becomes and will take longer and longer before being rideable.
After you have learned how do you lunge a horse, you will be a better prepared horseperson.