Creating a self-sustaining settlement, though, is even harder than creating a return rocket.
As a first step we should see if we can create a self-sustaining colony in Antarctica.
A self-sustaining settlement on Mars is not that difficult. Mars is not nearly as barren as the Moon.
Creating a self-sustaining colony anywhere is hard. Especially, if it has to work the first time.
You can stockpile supplies before humans get there. It doesn't have to self-sustain for as long as the supplies last. You can also send machinery to extract materials from the environment.