Instructions
Luigi Circuit GCN
Select "Time Trials" on the menu screen, followed by the "Luigi Circuit GCN" track.
Perform a U-turn, positioning your kart so that it faces the yellow line, but a little to the left.
Use a mushroom power-up to speed off and press "R" to jump. Your kart will go through the invisible wall and fly back on track.
Rainbow Road
Choose "Time Trials" and the "Rainbow Road" track using any character.
Drive to the twisted part of the track.
Grind against the right wall of the track to slow down. This should make your kart rise off of the ground and fly to the end of the twisted area.
Waluigi Pinball
Select "Time Trials" followed by "Waluigi Pinball."
Wait until a ball enters the area.
Press "R" to jump in front of it. It may bounce you onto a flipper, which will send you flying.
Move the control pad to tilt your kart as you fly.