Downhill is the 19th Advanced stage in Challenge Mode and World 3-9 in Story Mode.
A long starting platform leads to a long staircase down to a ring-shaped platform. There are 5 ring-shaped platforms connected by ramps. Each subsequent ring is lower and narrower than the previous. The goal is on the lowest and narrowest ring.
There is only one goal, the blue goal. Roll down the stairs at a moderate speed and slow down when you approach the first ring. The ramp that leads to the second ring is behind you and to the left. Follow the ramp and circle the second ring 180 degrees to get to the next ramp, and so on until you get to the goal. Slow down as you get to the narrower rings to avoid falling off.
A faster and more stylish way involves rolling along the side ledge of the starting platform and falling at an angle into the goal with good timing. You can also fall off the top of the staircase and use the map to land onto the lowest ring with the goal.