it all depends what gas the system uses in your car, if its R12 its a pro job as it needs to be recovered ( bad for environment, only bad for you if you are in a small space and release it as it dosnt contain oxygen and you can suffocate) if its R134a then its perfectly fine to do it yourself, no need to recover the gas as it is the same gas that is used in air horns, etc. R134a is the gas that basically replaced R12 after they put restrictions on the use of R12 and other refrigerants harmfull to the environment.
hope this helps,
Owen