Let's start at the very beginning, for that's a good place to start:
Start by getting your horse to move around you on a circle at the end of the leadrope. Begin by facing your horse. If you want your horse to track left have the tail of your leadrope in your right hand and your left hand positioned 5 or 6 feet from the halter. Point to the left with your left hand. Your goal is to have your horse move off in the direction of the point. If that happens, great! Drop your hand down to release the pressure. If not, continue to point, start twirling the tail of your rope, and advance (walk) towards your horse until it moves to the left and forward on a circle. Some horses might require you to hit them with the end of the leadrope to get their feet moving. Aim for the shoulder. This is termed "firming up". Don't be afraid to do it when necessary, just be sure to always offer them a "good deal" first (pointing). If you always offer your horse the point, it won't be long until he recognizes and understands the cue. Be sure to work both directions. Remember the point signals the direction, there is still slack in the leadrope. Think "Drive, not Drag" drive the horse with the tail of the rope, don't try to drag it around the circle with the leadrope.
Once the horse is circling, be working to acheive a lively energetic walk. Many horses will be dull and simply plod along without giving you any effort. A couple walk/trot transitions will help liven these horses up. Do what it takes to get these horses trotting, but don't have them trot very long- go back to the walk. Other horses will keep trotting around instead of walking. Don't let these horses trot more than 1/2 a circle. Keep changing directions with them until they relax and walk.
It is important that you walk forward on a small circle while your horse moves around you in a larger one. Make sure you're not walking backward trying to maintain space between you and the horse. This will cause you to move more than the horse- then the horse is groundworking you! If your horse is crowding you, use the tail of your rope to drive him off your space.
Also notice how your horse is bent when he is traveling on the circle. You want your horses nose slightly tipped toward you. But many horses travel counter bent- looking to the outside. If this is the case bump the horse's nose towards you while you drive the shoulder out. Give the horse some slack (release of pressure) when it is shaped up correctly.
Practice counting the rhythm of your horse's walk and learn to recognize the cadence of his feet: left hind, left front, right hind, right front. This is an important skill that you will build on forever- as long as your forever involves horses, and we sure hope it does!