Firstly you'll need to unlock The Flatliner Event event by completing all the other event and their tracks before hand. One they're complete with what ever medal you achieve, the event and its extreme tracks will be open to you.
I recommend you start on the very first track, Truth is Out There and you use the Phoenix 250cc. Just because it's the first one, doesn't make it the easiest but you have to start somewhere. The track overall is similar to a track you'll find in the hard events, but it has a few little trickier areas.
Remember that all you have to do is complete the track regardless of faults, time or medal. So don't go rushing in there if you're still learning the ropes. If you do find any of the other tracks a little easier, feel free to try them rather than the one mentioned above.