Simply load up the map 'Cortex Castle' and near the end of the lap before the big jump you drive around a clocktower, instead of driving around you'll want to take the shortcut, I actually fluked it on my first go then realised it was a bit tricky to get and i'd only hit it 1/3 times and what I learned was that it's best come from a wide drift (boosting of course) and head towards the inside left-hand side and then trick jump to make the shortcut.
I created this video to help:
Hope this helps.