Lye soap is caustic soda or sodium hydroxide mixed with any animal fat or oil. When mixed it reacts with the fat and gets hot. You stir it and pour into molds and hardens in a day or so. Easier than making a cake. Follow recipes for cold process body soap and put whatever kind of oil or additives you want during cooling process. Old school lye was just the sodium hydroxide and pig fat or Crisco. Wear gloves and eye protection the sodium hydroxide is the same used in drain cleaner and from experience it will burn you and is not good to inhale fumes when mixed.

Want to thank catfish Lynn and Jsouther for info

Did a little bit of google-fu and found this instructions for soap making.
https://frugallysustainable.com/how-to-make-lye-soap/
Also...
https://www.catfish1.com/threads/how-to-make-soap-bait.4211/
Hope it helps.