Level your body to the slope, swing on the plane of the slope and turn your body through
Many slicers hit the ball to the right simply because they are lined up to the right, when they go to hit the ball their mind knows the target is to the left and then subconsciously changes the swing to outside in so that the club travels towards the target causing a slice
Imagine pouring a bucket of water on the green and where it would flow is the break