Go lunging with the right equipment: splint boots with bells, a tight-fitting halter, a light lunging whip, and a lunging line you can comfortably work with. The boots protect the horse's legs when overloaded. What you don't need is for your horse to get injured and then remember that experience as negative. The lunge whip, by the way, is only an aid and not your main training tool.
Try using the lanyard in a round pen or an enclosed space with rounded corners and flat ground. Remember the goal is to maintain control of your horse at all times, so don't attempt in an unfenced area.
When you begin your lesson, always start with one direction and stick with it. If you choose to work on the left side, always hold your leash in the left hand and the excess line in the right hand (as well as the whip, if you choose to use one) and maintain all body positions identical.
You start on the left, pointing to the left, pointing the horse's nose to the left and moving your feet, swinging the end of your rope (or raising the whip) to the horse's hip to ask him to to lend. If your horse continues to face you, lead him forward by swinging your rope or whip toward his shoulder.
Ok, it's done, your horse is moving in a circle to the left. Follow it, staying in the middle of the roundabout with a sufficient line. Keep the whip low, or the end of your rope low, and walk quietly.
What you want to teach the horse is that if YOUR feet move, his feet should move too. Keep your body behind its withers to ensure forward movement. If you stand in front of the withers, your horse will stop.
If your horse stops, calmly continue forward and touch the end of the whip or rope to his hind legs to move him forward until YOU stop your feet. The main reason this approach will work, and work well, is that it's totally natural behavior for a horse. Watch a herd in a field and see what happens when one stamps its feet and walks away.
The others follow, they don't stop to ask questions, they go, and go at the same speed as that set by the horse in the lead in the hierarchy. If you lunge like this, your horse will understand your body language almost immediately. First the body language, then add the voice to the body language.