When you are at the end of the level in time trial mode, when you see the three boxes and the arrow pointing down the ramp, do not follow it. Continue in the way you used to reach it until you see the green gunk and the path. Go on to the path (hold A and use the D-pad) until you see the end with the ramp below. At that point still go forward you will be guaranteed to land on the ramp, only if you do not make a mistake.