Topdown maps of Moonview Highway

Due to the fact that turning against the drift makes it take longer to get the mini-boost, I tend to drive these karts and bikes exactly like the N64 version of Mario Kart, where you had to move the stick from side to side to get the boost.

1. Initiate turn and drift.
2. Steer against the drift.
3. Briefly steer back into the drift to get the boost.
4. Repeat 2 and 3, if necessary, depending on how tightly you need to turn and how quickly you want the boost.

From the video, it looks like the player does a trick off the final mushroom in the cave to get a boost, does a wheelie, then, just before he hits the grass, uses a mushroom boost. As he's in the air over the canyon, he steers to the left.

Keep trying! You'll get it.