Start on the multiplayer Crash map.

Go to the buiding with the raised car. Go to the front of the car (or back).

Then jump and press A when you're touching the bumper and you will be mounted on top of the car.

From there you may stay on the roof of the car or you can mount yourself on the sheet metal or the wood in the corners.

If so, put yourself next to a beam (also facing it), then press A.

This should mount you on the beams. From there, jump to either the sheet metal or wood platform.