Lots and lots of micro organisms in balls of dried soil samples add water and kick start the production of oxygen and co2.
The trick would be maintaining the environment and introducing biology at the right times. It just would not work if it was only humans that lived there.
We need to know what chemicals/life? is already there first though. Also it is my understanding that there would be a need for more co2 in the atmosphere to thicken it and raise the temperature. So lets get the planet breathing for Mars.

I think we should also change the Venusian atmosphere using a chain of micro biology to consume the sulphur and, ultimately, by-produce oxygen.
The atmosphere could then be "farmed" for use on Earth/Mars.