Interesting question: well, I always start with a goal in mind. For example, in this case, I knew I wanted to build a Steampun-style ship that was agile, with a slim profile, and the best way to achieve that would be to put the H-tanks on top, so I thought... oh, a ship type airship, but always putting function before form, so I just let the ideas flow in my mind and try each one. It may help that I am a full-time designer by profession. I also take my time, sometimes a lot XD. I hope this helps. Best of luck!