In the short term, you are either strip-mining soil to collect ice crystals, which you melt and store inside, or drilling a well (and piping it inside). You would grow food inside pressurised tents.

In the long term, mars lost its previous atmosphere over billions of years. If you have the technology to rebuild an atmosphere in human timescales (few hundred or thousand years) the atmospheric loss is small enough that it almost becomes a rounding error. Maintaining it should be easy.