Practice, practice, practice. From our Hidden Challenge guide
Unfortunately, this challenge is a lot harder than the BMX challenge, and there isn't any specific route you can take. The best advice I can give is to head around the top first, collecting the checkpoints at ground level, then get all of the checkpoints at the bottom of the dock. From there it's just really luck as to whether you hit the jumps at the right angle and manage to fly high enough to grab the rather frustratingly placed floating checkpoints. Going up the steep ramp slightly sideways and jumping alongside it, rather than up it seems to be the best way to go about getting the checkpoints, but even with full bike skill, it's extremely hard to work out while angle to approach at, when to lean back and forward, and even how much speed to carry. My best, and frankly only advice is to just keep trying the jumps until you get them right. Collecting the top and bottom checkpoints should give you about 1 minute to get the last 7 or 8, so that's about one every 10 seconds or so, which is usually 2 or sometimes 3 jump attempts.