Mankind can work far faster and efficiently than nature. From what I've read, the early Earth underwent several phases in the development of its current atmosphere. From a CO2 to a methane, to a nitrogen/oxygen based atmosphere. We could speed up the terraforming of a CO2 atmosphere by bypassing the other steps with CO2 consuming algae. Imagine a planet with large bodies of water that had yet to develop life. With no other organisms consuming it, the algae would spread like wildfire. Its growth spurred on by the CO2 enriched atmosphere. Once the algae began spreading, plankton and various types of bacteria could be released, followed by kelp and other forms of aquatic plants. Within a few decades, the waterways of the planet would be turned into massive oxygen producing factories. This process would be greatly sped up since there would be very few oxygen consuming animals present.