What isn't said is that water vapour is a far more efficient green house gaz than carbon dioxide. But first thing first, we need to engineer a bacteria that will survive in the harsh environment of Mars and grow into its soil, to release frozen CO2 and water. Once enough of both are present in the atmosphere, we'll be able to introduce plant that survive harsh, desert conditions. Finding the right bacteria is the only tough thing. Once we have it, seeding it is a matter of sending a rocket to the red planet. Once started, the process would have to reach a point where it can sustain itself. Its a matter of starting back the ecosystem that once rose began on Mars. Having the global temperature of the planet being elevated by only 1-2 Celsius degree could melt some more underground water, adding to the green house effect. This wouldn't be instantaneous, but it would mostly be done free of any mankind intervention except at the beginning. After a couple hundred years the condition will be fair enough to introduce plants and bugs, the air would start to become breathable. Its a long term process, but it can be done, fairly easily. The "gardening" task wouldn't have to be accomplish until many hundred years, and we can hope some technology to get to Mars fairly easily will have been invented.