The only thing you need is a ship in orbit of a landable planet with one crewman onboard that ship. To reach this state, you can take off with one crewman, acquire a second ship and put your captain on it, or move your crew from your captain's ship to another ship already in your fleet.

Now that you have your captain-only ship in orbit, press P and select any landing site. The achievement will unlock when you touch down. You do not need to leave the ship.

If you used this guide to achieve All Alone, please rate it up.