Take some Mars-like soil and atmosphere.
Prepare the soil by actively pumping out all gases already stuck in rocks of similar composure. High vacuum would make them evaporate fast.
Since Earth plants would contaminate a finite closed environment with what they make as a by-product of living processes, plenty of soil and atmosphere are needed for a few generations of alive plants.
Train a bunch of plants along with their typical symbiot microorganisms which are by default eager to convert any little bit of sunlight into molecular chains and oxygen - for example, the plants at the bottom of the hierarchy in a jungle, which receive 1 or 2% of the total sunlight of a bright mid-day.
This gradual evolution by training is done by partially exposing the plants initially through reduced pressure of the atmosphere, and later through changes of its composition, and expected sun/star light conditions, while replacing the soil partially. The aim should be to impoverish in order to emulate Mars as a planet.
A couple of generations in, after plants have learned to live in reduced atmosphere pressure, send the most prolific faunas directly on Mars. It may be necessary to expose them gradually to Mars, because they are trained for it, but are babies at it still.
A backup plan is necessary only to pre#FUR civilizations. Let's get to post#FUR!
Enjoy April 2020 !
The #FractalUniverseRevolution is Unstoppable!