Thanks to the video provided by mhsjlw, I was able to finally figure out how to earn this achievement. My problem was I wasn't using enough sticky bombs and thus wasn't getting enough force to propel myself the distance.
Unlike the video, I earned it on Dustbowl, by flinging from the top-right entrance to BLU's spawn, to the cliffs in the diagonally opposite corner:
Place your stickies about here, and aim for the cliffs. Remember to jump and crouch midair.
It took me a while to get an action shot:
The achievement should unlock just as you hit the cliffs. (I didn't even die!)