I have been doing this as a habit since I already do it with my lotions and hair stuff... I have a notebook with all my recipes, and after each recipe, I have blank pages where I put the date and notes about each batch -- if I changed an ingredient, a scent, a process, etc, and room for later follow-up in case it turns out really good! Or if it has issues...
I only print labels one batch at a time, so it's easy for now to put it right on the label. When I start getting bigger and start selling the soaps too, there's going to be a lot more though, so I may have to rethink it. But for now, simple is good.