Terraforming is largely about taking what's already there and rearranging it so it can support life.
Then you're never going to terraform Mars. Its native nitrogen and water resources are very limited. Getting a sufficiently thick atmosphere would require very large nitrogen and water imports, and I don't mean sci-fi's popular, "hit it with a few comets."
Native Martian water resources would cover the surface to a depth of about 6 meters. Concentrated in the likely lowlands - the North Polar Basin that covers 40% of Mars' surface - and you get an "ocean" of 15 meters depth. To fill the North Polar Basin to 1 kilometer depth (a useful ocean size for supporting a global water cycle), you'd need another 58 million cubic kilometers of water. That's a sphere of water about 480km in diameter, or more than the entire water content of Enceladus or Ceres.
Nitrogen is similarly in short supply. To get an Earth-like atmospheric composition for a world with 28% of Earth's surface area, you need to import about 28% of Earth's atmospheric mass of nitrogen. (That's ignoring the difference in scale heights due to lower gravity.) Since Earth probably doesn't want to share then you'd need to remove about 24% of Titan's atmospheric mass or 9% of Venus's nitrogen.
A one-stop source for Martian terraforming might be Titan. Get some von Neumann robots running wild and you can strip-mine its crust to about 1 kilometer depth for sufficient water, liquefy a quarter of its atmosphere for the nitrogen, and start flinging the goods to Mars by mass driver. If you're thinking of terraforming Venus, it has excess nitrogen (three times' Earth's inventory) and plenty of spare carbon that would be useful on Mars.