How do you back-flush the heater core in a 1992 Nissan Maxima?

This sort of question is a little involved, and essentially cries out for pictures and diagrams, which we can't do here.

Your best option is probably to buy a service manual... any good auto parts store should have these, and they typically cost 15-20 dollars, a pretty small investment compared to labor costs at a mechanic. They'll have very, very involved instructions, with lots of pictures to help you.