High output pickups and your amp turned up really loud.

Didn't he do stuff like tapping on his guitar neck/body to get feedback too?
Essentially, just mess around with your gain up high.

Oh and also, watch live Nirvana videos! Haha, I used to be a Nirvana FREAK
watch him at the VMA's 1992 listen to the feedback at the end of lithium,EPIC.