Its Super Easy... This is how to do it.
Not sure how to describe it, but when I am riding I do these steps just a split second apart. Lets say I am trying to turn left. I start riding with two hands! You can do it with one hand to start but its way harder! LOL and you can really confuse your horse.
1. Right leg aid
2. Lift right hand, and bring over wither (this places the right rein on the neck)
3. Left direct rein (lift hand pull up and away)
I prefer a horse that can be ridden with just my legs. So all and any horse I ride it goes in the order of following... Voice, seat, legs and reins.
I always ride like this on a green horse, and they pick it up in a week or so with a few hours of riding each day