To unlock this Achievement, you need to complete all the game missions *and* the Jet Graffiti / Technique / Crush challenges. There's one each of those challenges per area, meaning an extra 15 missions to clear.
(Note: in case you haven't noticed it, you access the Bantam Street / Grind Square areas via the pink "To Grind City" icon on the lower left of the map/selection screen.)
This Achievement will unlock when the final mission has been successfully completed. If you're diligently getting your Jet rankings, you'll unlock "When You're A Jet" at the same time.