If you had an air-conditioned car that was being scrapped, so you could take all the bits off it then you only need time and a bit of work.. but if you don't have the parts then you'll need to get hold of them and it will take time and a lot of cash, compared to the car's value.
A/C has an extra radiatore (condensor), extra pipes for the a/c system (and clips to hold/route them) a pressure and cabin temperature sensor to wire and align with the Body Control Module Ahhhh Ahhhh a compressor pump with its own pulley (you'll need a longer aux-belt too) and the heater matrix is different (for the additional plumbing).
And you'll need a new dashboard panel with a switch for the a/c. :chin:
It's this last thing that puts everybody off doing the conversion...