You probably want to do this on an empty server to have enough time to get a long jump. So just open the console and start a local server on your pc with the command "map pl_badwater; sv_password 1234; maxplayers 32" (without quotes). I used the map Badwater Basin for this achievement because it has large open areas which is perfect for this jump. Quick note: This achievement can be done on any map which is big enough. Well (ctf_well) works too.

This will take some time. If the server is started, pick the blue side and obviously the demoman. You can also use "mp_waitingforplayerscancel 1" (without quotes) to get rid of the waiting time before the match starts.

So lets go and get your achievement ...

A good place for the jump is just at the blue starting area (BLU Spawn) in Badwater Basin in front of the cart. First of all you need to place one Stickybomb on the left and one on the right side in front of you.

After that you just walk between these two Stickybombs and if you just pass them (because you want to the a long jump instead of a high one) press jump (default: space), crouch immediately after it (default: ctrl) and then alternative fire (default: mouse 2) to release the Stickybombs. Dont forget to stop crounching before you land. Achievement done!

You can also watch this small video