Long-lining means to ground drive your horse while walking beside or behind him. It's an important step that can't be omitted or rushed. Take your horse to your safe work area and put him in his surcingle. From this step on, I use a home-made "work bridle" consisting of a bridle with browband and side checks but nothing else. See the photo for details of this bridle. You can also use a blinker-less driving bridle, but I strongly recommend that you don't use blinkers yet. I want your horse to see and get used to everything around him, including you and the swaying long lines. You'll want to fasten the side checks VERY loosely, just enough to keep him from dragging his nose in the dirt. I start all of my horses in a french link bit, which I feel lies better in their mouth and won't poke the roof of their mouth when rein pressure is applied. For horses that continually get their tongue over the bit, I'll switch to a copper mullen, adjust so that it causes a nice wrinkle at each corner of the mouth.
My lines are made of either light-weight rope or flat 1 1/2" wide nylon and they're extra long. 12 feet is a good length, allowing you to stay a safe distance away from you horse and giving you plenty of extra rein to play out in case your horse tries to pull away from you. Note that I recommend using a surcingle or crupper-less harness for this step. If your horse gets confused and winds himself up in the reins (which he likely will), you don't have to worry about him breaking your crupper. The worst that will happen is that the surcingle will slide forward onto his neck or withers. If that happens, simple reposition it and continue with the lesson.
Before you start teaching him to ground drive, test his responsiveness to the bit by flexing him a few times to each side. When he's giving his head willingly, run your long lines through the lowest rings on your surcingle, step several feet behind him, and cluck.
He's not going to understand at first and will likely try to turn to face you. Keep your lines low so that the one coming from the far side of your horse drapes around his hindquarters just below the hip. That way, when you pull on that rein to turn his head away from you, the pressure on his hip will help him to understand. At first, you might only get a step or two of forward motion before he tries to turn toward you again. During those few forward steps, keep your rein contact very light as a reward. As soon as he tries to turn to face you again, pull one rein to turn his head away again and cluck.
The other response you might get is for him to try to run off. Hold on firmly and steer him toward the fence while saying "whoa." Don't yell it and don't panic. You're in a small pen and can easily stop him, even if it means bumping his nose into the fence. No matter what it takes to stop him, let him settle for a minute then cluck again. Today, on this first attempt to long line, you need to succeed. And you can. Just don't lose your cool and give him time to figure it all out.
Eventually, whether it takes 5 minutes or 30, you'll get him walking around the pen with you behind him or slightly off to one side. Once he's going forward freely, let him. Don't try to stop or turn him just yet. Let him adjust to the idea of walking away from you. After a few rounds of the pen, gently pull on the outside rein and ask him to turn into the fence. Keep the line low so that the pressure on his hip reinforces your command. As soon as he commits to the turn, relax the rein pressure as a reward and cluck, asking him to go around the other way of the pen. Let him go a few times around then pull the outside rein, down low still, turn his head toward the fence, and ask him to turn. When he's moving freely, you can either remain behind him or step toward the inside of the pen, keeping your inside rein shorter and your outside rein longer.
I'd do this at least 2 or 3 times each direction - enough to make certain that he's starting to understand but not enough to frustrate him. Now, you're ready to teach him to stop. Adjust your position so that you're behind him, tell him "Whoa," and gently apply pressure to both reins. If he doesn't stop, turn his nose into the fence and tell him whoa again. Eventually, no matter how long it takes, he will stop. As soon as he does, instantly relax your rein pressure as a reward. If he starts forward again before you ask, repeat the whoa command and make him stop again. Do not jerk on the reins or raise your voice. Just work steadily and quietly until he comes to a complete stop.
Let him settle then ask him to go forward again. This time, let him go only halfway around the pen then ask him to stop again. Repeat this a couple more times to be certain that he's starting to get it.
Before you stop for the day, we're going to teach your horse to back up. With him standing quietly, gently pull backwards on both reins and tell him "back." Do not try to force him back or jerk the reins. Just take a firm feel of his mouth and hold it there. Don't keep increasing the pressure. More than likely, after a minute or two of enduring the pressure, your horse will move one foot backwards. As soon as you see the foot starting backwards, release the rein pressure. After a second or two, apply the same amount of pressure again and say "back." As before, as soon as one foot starts backwards, release the pressure. Try to get 4 to 5 steps backwards, one step at a time if needed, before you quit for the day.
Some horses will "lock up" and just brace against the rein pressure without backing a single step. If your horse does this and simply refuses to move after several attempts, then another plan of action is needed. Gathering up your reins as you go, step in front of him. With one hand on his the front of his nose and one hand gently pulling both reins backwards, tell him "back" and make him take at least one step. Stop all cues, praise him, then ask for another step the same way. Practice this for 4 or 5 steps. Tomorrow will be soon enough to try backing him on the long lines again.