Hold them evenly high, elbows next to your body, wrists straight so you can draw a straight line from the horses mouth to your hand. Keep them upright, thumb on top of the reins. I always have my students check their hand position at H, no matter what exercise, show or training.
hold 2 whips. they should cross in the middle of the horses neck. that is the correct hand position. keep a straight line from your elbow to the bit. hold books between your elbow & side to help keep your elbows in
Elbows in, Hands together and thumbs up! Sounds easy, but I always end up popping my elbows out forcing my hands apart and thumbs down.....my trainer used to make me hold a ruler with my reins...that helped.
Really concentrate on it. At a point in the arena (like the corners, or at E and B) check to see if they're in the right spot. Or set a timer/alarm to go off every minute and you can check then.
After a while you'll realise you'll have your hands in the right position more and more. Then before you know it, it will become habitual.
Try and "check" that your hands are in the right position without looking, so you can get the feel of it.