I'll offer my opinion, others may have a different approach that will work for you as well. I try to start them as yearlings, mind you not on a line, but in hand. I do have a round pen, but you can use a small paddock. I use body language as well as the english word. I take the young horse into the round pen on a lead, they are on the rail, they seem to work more easily on the left, I say walk, then walk, I teach them to trot, I say the word trot, then I trot, if they don't respond I flick the lead toward their rear until they trot. I stop praise, then walk, then trot, lots of praise. I also teach the word whoa, if they don't respond I take the lead and tap their chest till they stop. I continue this for however long it takes for them to associate the words to the actions. Lots of positive reinforcment, and praise, and patience. Then I teach in the other direction. After this phase is down pat, I turn them lose, I hold a lunge whip in the right hand then tell them walk, usually they walk, if not, I crack the whip just a little, sometimes this causes them to trot instead of walk, so I will say trot as soon as they do it, then I'll say whoa, lay the whip down, usually they will stop and look at me for praise, which they get. I don't progress to the lunge line until they have all this down pat free, and it usually takes a while, and then I only use the lunge line at shows, not at home. When working the other direction put the whip in your other hand, you may need to actually use your body to sort of lunge to get them to move off the off side. Pretty soon you won't even need the lunge line, just your voice and body language, Practice makes perfect. I need to add, some horse just won't get the hang of it unless you plan to spend every day with them. We bought a proven show gelding, AMHA World Reserve Ch. that was never taught to lunge, I don't care want you do he won't move, not a whip, not a shaker bottle, no amount of chasing, nothing, he just stands there and looks at us like we are crazy, but he was older when we bought him and set in his ways. Good luck, hope I could help in some small way.