Two minutes, two days, two weeks

Rule of thumb: You die of oxygen deprivation in two minutes, die of dehydration in two days and it takes two weeks to starve to death.

That's not actually accurate. It's just a rule of thumb that gives you some idea of the order of magnitude of these problems.

If you want to colonize space, you need to solve for making sure people can breathe and get water locally or keep a water supply on hand that will get them through until the next supply ship, but most people will not starve to death if deprived of food for a few days so you have some latitude on solving that problem.

We can ferry food up to the Moon and as long as there's oxygen and water, we have a week to deal with any dire problems without worrying anyone is likely to die of starvation. That means we can colonize the moon while figuring out how to establish a local sustainable food supply.

You can't ferry food to Mars. It's too far.

A Moon colony is very doable. Lots of latitude for messing up without it killing people while we figure it out.

Then we need one or MORE colonies in that debris field that failed to become a planet, the asteroid belt. Multiple colonies there gives us options for leapfrogging to Mars and extends the launch windows which are constrained by how long it takes to get there because of current positions of planets.

Someone at NASA could run calculations and pick out potential candidates of large asteroids to colonize and then have their entire week eaten by crunching numbers on launch windows to Mars if we have three or four well placed intermediary stations.

Mars probably needs to be seeded before we send people there. We probably need to start terraforming it and establishing plants there that grow on their own and produce oxygen years before we try to ship a human there.