LOL.... did you ask Me that with a straight face ?
The only way to do it right in my opinion... is to flush, replace the orings with new correct color ones, smear them with the correct color Nylog when reinstalling them... measure the correct amount of fresh clean oil into it at the right time.... weigh the correct amount of R12 into it....
After , of course, doing the very best evacuation job you can do....after checking for leaks with a vacuum AND with pressure ( if you have the equipment ) by pressurizing with dry nitrogen before committing your R12 to the system...
But of course I am an old fuddyduddy about this stuff... pretty much believing what I read in the three AC manuals ( including the MB one ) I have.... and adding up the numerous problems encountered by people who leave out one or more of the steps which may be required to get it Right/Cool for the LONG HAUL...