To unlock this achievement you will need to find and successfully complete all the stunt jumps in Liberty City. There are a total of 50 jumps that vary in difficulty. The best advise to completing all the jumps is to change vehicles when you are having issues completing jumps. If you do plan on using the motorcycle (and you will have to for some of the jumps) you have the ability to shift your weight with the left thumb stick so that you can hit the landing without crashing. Below is a link to all the jump locations in Liberty City and the blue markers also show you the direction at which you need to approach each jump: