The world order went down and humanity was pushed back into an post apocalyptic society (no nuclear fallout). Most of humanty vanished. Only the old ones can remember how it was to have electricity. (6-7 generations)
Right after the collapse, most things were gathered such as food cans, drinking water etc. Over the time, people got independent from this gathering in terms of food, metal works, construction etc. The biggest cities have about 50.000 habitants.
A mid aged man, which never saw humanity at its peak, remarked that the supplies found went to a minimum. In his childhood, food was plenty compared to now, but people found a way to work around that. The only thing he misses is hygienics. People forgot how to handle waste, disease and filth.
He knows he can't change the world, but wants to start somewhere. So he decided to reinvent "soap". (It's not like he knows how to and the name, but it's about something so people are (easier) able to clean them self and their surroundings)
The question is, how to produce Soap in such a setting? What could happen, so he gets the first clue, how to do it?