How would you go about replacing the serpentine belt tensioner on a '93 Asuna Sunfire?

That's a fairly unique vehicle but the motor is basically the same thing they put in '90 - '93 GEO Storm and/or Isuzu Impulse. You could take it to a qualified ASE certified dealer and tell them I sent you. Or, if you want to tackle this yourself, you'll need a repair manual to walk you through all the steps. You should be able to pick up a book from the same parts store where you can buy the belt tensioner.