How you should hold horse reins depends upon the horse you are riding. If you are riding someone else's horse, you should ask if the horse in neck reined or is direct reined. Direct reining is when you take contact on one rein to turn a horse in that direction. A neck reined horse, however, turns in response to the rein being laid on the opposite side of the neck. (For example, to turn left you would lay the rein on the right side of the neck.) Each method of reining a horse has a different way to hold the horse reins.
How to hold the horse's rein when direct reining: When riding a direct rein horse, the horse's reins should be held one in each hand. Each rein should enter through the third and pinky finger, and come up through a loosely held fist, exiting between the top of the index finger and thumb.
How to hold the reins when riding a neck rein horse: The proper way to hold the rein on a horse that knows how to neck rein, is to pick up the reins with your index finger between the two separate reins. Allow the leftover reins to come down through a loosely held fist. When holding the reins of a neck rein horse you should ride with one hand only. The back of that hand should be face up and the palm of that hand should face down towards the saddle