Elon Musk has published an in-depth, 15-page document setting out his vision for a self-sustaining one-million person colony on Mars. The manifesto, entitled Making Humans a Multi-Planetary Species and published in New Space journal, is available until early July.
One of the key tenets of the publication is that regular people should be able to afford to make the trip to the colony should they so wish. To do so, massive efficiencies will need to be found and Musk identifies four core areas: the majority will come from rocket resuability and the rest from refilling in orbit, propellant production on Mars and choosing the right propellant.
You can read the official document here in order to get full details. It is certainly an interesting read for any engineer!