While you are playing the Balloon Buster event, you will notice a few star shaped balloons on the courses. These are very rare and on the Lv.1 event I have seen 2 of them. One is off a jump in the middle of the track and the other is off the middle of the big jump at the end.
What you need to do is go fast enough so that you get the distance and height to actually reach the star. To slightly complicate this a bit more, you also need to be doing a trick when you make contact with the star. I suggest doing a basic spin as these are easiest to pull off consistantly. So pick either a speed or all-around character, your best board and work your way down the hill. Try to complete a trick prior to reaching the jump that leads to the star so you will have the extra boost from a clean landing. Then lean forward to be going as fast as possible, line up and jump off the ramp toward the star. Start twisting to spin and keep your fingers crossed. This will probably take a couple of tries but you will know when you get it because the achievement unlocks before you even hit the ground.