Hi,
It's
doable as a DIY project, but generally, best left to the pros.
To do it properly, you need to drain the system, evacuate it using a vacuum pump (which boils off any remaining moisture and Oil), add a specidic amount of a specific Oil (ND 8 - 6.6 ± 0.5 oz.) and then add the refrigerant (R134a - 850 g (30.0 oz.)).
In theory, you should never need a recharge, but in practice, because there are so many joints in the system, an 'O' ring will usually dry up and allow some of the refrigerant gas to escape. A Shop can identify the leak and replace the 'O' ring(s).
Also, despite DIY kits being available commercially, US Law (EPA) requires that anyone servicing an AC system be certified in completing an EPA recycling course (available here -
http://www.epatest.com/608/ ). I have taken this test and can do a system. I own a vacuum pump, but lack the required recovery equipment, so I just leave it to the pros.
R134a was developed as an alternative to R12 (Freon) which was found to be Ozone depleting. Subsequently, it was found that R134a while not directly Ozone depleting, will combine with Sunlight and other atmospheric releases to form compounds which do deplete the Ozone layer and so should not be free-vented. A Shop will have the proper recovery equipment (assuming any of this matters to you). Good Luck!...
Happy Motoring!... Jim'99