Lemme see if I can explain it in words. I use this method (most everyone does, taught to me by a good friend who was taught by Buck Brannaman).
Going to the left. line in your left hand (your left hand is showing the horse which way to go, the right hand says how fast, ie, driving aid). Make sure you are lunging behind the shoulder, as you walk, you are mid barrel to hip.
To turn. Place right hand on line while you step towards and in front of the horses shoulder. This is important. Practice it without horse to get it right, it can become complicated if you are not "handy" enough to co ordinate.
So, again, placing right hand on line while stepping towards and ahead of the horse (essentialy blocking forward movement), put pressure on the line with your right hand, pulling/bumping to the right. Ask very lightly to start, get bigger as you need. This is BIG, KEEP walking towards the horse. It may feel uncomfortable, but you must keep walking towards the horse until he/she respomds and turns right. Once the turn is made, use your left hand (now the driving aid) to keep him/her going.
It can be very wierd feeling, like you're not in sync, but practice and you will get it. Once the horse does as you ask it is important to STOP the pressure.
Do the opposite to switch direction again.
I learned the most amazing thing at a clinic last year, I can actually walk forward, while my horse is in front of me going slightly backwards, switching directions whenever he hits the side of my body. Takes practice, and I am not perfect, but man of man you should see those rollback turns!
I see my thighs have grown since the last time I wore these breeches........