I tried googling it, and it says nothing $^&@ing helpful, just posts saying like "There is a glitch sc on Bowser castle wii ZOMFG" VERY HELPFUL

Ask someone who can do it to make you a marker ghost.

It's hard to explain, you just have to watch the early WR's and get the hang of it.

It's actually not that hard, just learn the right time to drift in to the thing, or go straight into the wall.
Use Mach Bike if you can't do it with Bowser Bike. xD
Use shroom when you are going through the wall and when you think you can't reach the track.

Or do the non-glitch way.

Lol it's hard enough with shroom and mach bike D:
Thanks for the vid I'll watch it now

No problem.