>step one
crash a asteroid made of water with a mas of 5x10^17kgs on mercury
>step two
do nothing, within a few decades 90% of the water from the asteroid has been photolysized by particle and uv radiation into hydrogen and oxygen with the hydrogen being lost nearly straight away and a 0.3 bar atmosphere of pure oxygen being lost over a few thousand years.
>step 4
you now have 3 million square kilometres at each pole with a temperature less than 50 Celsius but higher than freezing with remaining ice in a small ice cap in the permanent night cold trap poles.
>step 5
over decades slowly launch solar shades at sun-mercury l1 and expand the habitable regions of mercury
>step 6
profit?
you now have a planet with the same gravity as mars, with a breathable atmosphere and radiation shielding from said atmosphere plus the small magnetosphere of mercury, it has massive reserves of metal, and a surplus of energy abundance (solar at the equator 9x more energy than earth).
sure the habitable region is only 5-10% of the planet but there is the option to expand that with solar shades and the atmosphere can be topped up every few hundred years