How do you make soap in medieval times?

In medieval times, soap was made by combining animal fats, such as tallow or lard, with ashes from wood, which contained lye (sodium hydroxide). The fats were rendered down and mixed with the lye to create a chemical reaction called saponification. Herbs and essential oils were often added for fragrance and antibacterial properties. The mixture was then poured into molds to cool and harden before being cut into bars for use.