1st get a bike any 1 but sanchez or NRGs are best
2nd go to the beach on the 1st island
3rd ready up at the far corner of the beach and full speed into the rocks
4th if u hit the right rocks u should fly into the air
5th you are then able to do a flip in the sky forwards or backwards
6th if you keep going u can manage to make 3 flips i know this becuase thats all i can manage to get tho i nearly made a 4 backward flip bt landed on my face lol
Hint
if you are doin it it doesnt count if u land on your face u need to land it