In a double boiler, over medium-low heat, add the grated soap and the water (starting with 1/2 cup). Slowly melt the soap and mix as you melt. Add more water if needed. You want the soap to melt but you don't want to add so much water that it won't harden again.
Once the soap is melted, remove from heat and let cool slightly.
Add the oats, vitamin E, essential oils, and herbs, if using. Mix well.
Before the soap sets, pour the mixture into the soap molds.