This is one of those self explanatory achievements but I'll make a guide in case someone's struggling. To get this achievement make sure you're in "Rainbow Cliffs" (Basically the Hub of the game) and head over to the little bridge connecting Bli Bli Station to the little island. Walk onto the bridge about a third of the way and jump off of it. Make sure to not hit the water as it will break your fall. Do this 4 times and Bam! The achievement is yours.