Methods for Terra forming planets (Theories and Ideas)

I was thinking that maybe many different types of bacteria could be engineered that could convert chemicals in rocks or in the atmosphere (if the planet has one) into things we need to grow plants and survive on such a planet...and to create a useful atmosphere.

Here is my idea to Terraform Mars:

Direct an asteroid (around 50 miles wide) and slam it into Mars's south pole. I think it would melt most of the ice on the south pole releasing Co2 and evaporating the water which would lead to a thicker Carbon dioxide atmosphere and Co2 would trap the heat from the asteroid impact in the atmosphere and could collect more heat from the sun raising the temperature on Mars. Then we could build a magnetic field around Mars. After that, the temperature on Mars would be around 80+ Degrees F (if we did it right) and then we can grow plants and send humans there.

Sure ... that could happen.

But it would be the last step.

There are no planets in this solar system that could be terra-formed.
Mars is the best option, but its temperature, dropping to less than mimus 100 Celsius is way too cold for bacteria to grow.

It would need to be a process that warms the planet, adds water, and an atmosphere. ... Then bacteria could be added.

Don't expect a good answer because I asked a question on terraforming a few weeks ago and most of them were crap...