When the ball is below your feet it is actually a little farther away from you if you stood the same number of inches as you normally would with a given club. So consequently to get down to the ball, you have to bend down more.
How to Adjust Your Balance on a Downhill Lie
Now when you bend over more you are very dangerous that you might fall forward on your toes. So once you get in that position beside your ball make a practice swing, bend over and then sit just a little more to get your weight back so that you can maintain your balance while you are swinging.
The biggest problem people have here is they tend to lose their balance and they want to stand up and they end up topping or hitting the shot farther right. It will curve a lit bit right because of the slope. Just allow for that with your aim, do your routine, do your practice swing and stay out over the ball, staying balanced. You are better off to take an extra club here and not swing as hard. So that you can maintain your balance, balance is the key to that shot.