My brother has a 1993 SW2 wagon and wants to convert his A/C over to R-134a. He has the kit but doesn't know how to discharge the system of the old R-12.

Any other helpful advice I can give him?

the vehicle needs to be taken to a EPA 609 certified tech to be discharged and evacuated, special equiptment and training is needed. i also suggest having the tech retrofit and charge the system. the tech will then be able fully service the system if any other modifications are needed for the retrofit(IE - oil, seals, hoses) to prevent any problems later, as well as accuritly charge the system(the suggested way also requires special equiptment).

My brother has a 1993 SW2 wagon and wants to convert his A/C over to R-134a. He has the kit but doesn't know how to discharge the system of the old R-12.

Any other helpful advice I can give him?
I agree with above..take it to a tech. The tech will pump out the r-12 into a certified container. Then he should change the accumaltor and fitting. The fittings are not the same for r134.
Then he wil pump the system down to a vacumm to totally dry it out and then add oil and new freon 134.
Just keep in mind that a converted r134 system will not cool as good as the newer 134 factory systems.. but that is ok for it's like an ice box anyway in a saturn.