Having established a reliable route towards Mars, we use a series of six-month journeys to build the first habitats and set the foundation for the long-term terraforming process.
By melting the ice from the soil and frozen CO2 from the poles, we begin creating a martian atmosphere. Huge mirror satellites and factories on the ground would start this process, that only gets accelerated while the atmosphere traps more and more greenhouse gas from the Sun.
Once the planet's temperature is above freezing, we could see rain and flowing water on the surface or Mars. At that point, microbes, algae and lichens would help generate oxygen from the atmosphere.
Eventually, the atmosphere will be so dense, flowering plants and green forest will grow on Mars, enriching the atmosphere with even more oxygen.
At this point, Mars will have become our second home, and we will have established cities and other large scale structures.