How can this be done? We know life now exists on Earth. We understand
some of its traits. In fact, the whole idea that Mars may have once had life
is based on comparing evidence from a sample of alleged Martian rock with
what we know about terrestrial life.
The idea is to try to recreate Martian life through genetic engineering -- i.e.,
trying to make a lifeform having all the supposed traits of the alleged Martian
fossils found on that rock sample from Antarctica. This can be done either
as a thought experiment, a computer model or a physical experiment.