Opening the book…
On a slope your body wants to fight gravity, and that fight is what produces fat and thin strikes off uneven lies. Matching your shoulders and hips to the hill lets you swing along the ground you're standing on, which protects the strike. Once the strike is safe, the flight is predictable: the ball tends to move in the direction the ground falls away, so aim can account for it.
Set your weight and your shoulder line to match the slope — feel level with the hill rather than level with the horizon — and keep your balance by making a shorter, smoother swing. Ball above your feet moves right to left for a right-hander and flies a touch shorter; ball below your feet moves left to right and needs more club and a lower finish. Aim off enough to let the curve bring the ball back to your target.
On very steep lies, the priority becomes advancing the ball safely rather than any target line, so take a lofted club and play it into the widest space. On a gentle uphill or downhill lie with a short iron, matching your shoulders to the slope is often enough on its own without any aim adjustment.