This is one of the challenges you have to complete in campaign mode. What you want to do is stay in the orange lines most the time when you're turning curves because most of the skulls are in the outside of the orange line. Leaning is very important because if you feel that you are getting closer to a skull, use your lean to move in the opposite direction. You only have 1:25 to do this, so you need to be careful, but also speed up while your in the orange lanes. When you leave a corner, stabilize yourself using the stick to center yourself so you don’t hit the walls and slow down.