Shred the bar soap using a cheese grater. Use the finest side for quick softening; a coarser shred also works.
Fill a pot with water. How much water you use depends on how thick you want the soap --- it's not an exact science. Start with 1 qt. water, or 4 cups, for every cup of soap flakes.
Heat the soap and water to a slow simmer, stirring until the soap melts. Add more water if it's too thick; add more soap flakes if it's too thin. Remember, though, that it thickens as it cools. Add 1 tbsp. glycerin to help thicken the soap. Also add 1 tsp. of essential oil, if desired, at this time if the bar soap didn't have a fragrance.
Remove the softened or liquid soap from the stove. If it's too thick, add a bit more water and stir. If it's too thin for your liking, add more soap flakes and reheat to melt the new soap.