Horses have what you may call a 'driveline'. This 'driveline' refers to their shoulder. If you're behind their shoulder, you drive the horse forward. If you step in front of that shoulder, you drive the horse the other way or ask them to face you. This is how horses move each other around naturally within a herd, so if you step in front of that driveline with your body, your horse knows that and will turn in as a result.
It's fairly difficult to explain when you cannot see it demonstrated... You should be slightly behind your horse's shoulder when lunging to urge them to move forward. If you want them to change direction, step in front of that shoulder. I highly recommend watching some of Clinton Anderson's groundwork videos where he explains how stepping in front of the driveline affects the horse. I can provide a link if needed.
I hope you understand. If not, pop over a message and I will try and explain further.