Once a "Big Boost" appears in the arena, or it's been approximately enough time for one to spawn, Boosted Wall Climb up to the area shown in the image. There is a very small area in bounds that you can sit on.

Once the timer starts, drive out and right to the Highway (the road taken when the starting the map). Follow the Highway and take the small road (in the image) up to the upper tier of it. Only make turns using you camera or slight movements of the stick, the aim is to build up speed. Whenever your Airframe's engine begins to overheat, either use a boost to delay it, or let off on the engine slightly.

The banner to finish this leg of the race is on the other side of the bridge. Using a spare boost, make the jump off of the small wood platform (in image) and use the boost to pass the finish line.