The vast majority of runners perform the same movement for this star, regardless of skill level.
Press C-Down and C-Left upon entering the stage, hold straight left, jump and kick to land next to the slope's edge, then perform a triple-jump dive over the hill.
Landing closer to the slope before the double jump makes it easier to land on the back of the hill after the triple-jump dive, allowing Mario to gain more speed before sliding off, thus saving more time.