This achievement requires you to fire yourself from one of your ship's cannons onto another crew's ship deck. It may take a couple of attempts depending on if the other crew notices you (and sees you as a threat), or you fire and do not land on the ship's deck
First, find another crew's ship - you can sometimes see them when sailing into outposts, though any will do. Once you find another ship, line it up with one of your ship's cannons and drop anchor. If the cannon is loaded (there will be a prompt to unload it if there is), press
to unload it while looking at the end of it.
Next, look at the cannon's base and press
to set up the shot. Try align the other ship's mast with the centre of your screen. This may take a bit of practice, so don't worry if you overshoot.
When the shot is lined up, look at the end of the cannon again and press
to load yourself in. Press RT to fire.
Hopefully, you should hit the other ship's mast (you do not take collision damage from firing into things), which will act as a barrier, and you will land on the deck.
Video by Maka91:
If you struggle with this method, see Indiscriminator's comment.