Begin by measuring all four oils and placing them in a medium saucepan. Heat the olive oil, coconut oil, tallow, and castor oil over medium-low heat until they are fully melted.

While the oils are melting, measure the water in a food-grade plastic bucket or a stainless-steel bowl, avoiding glass to prevent cracking from the heat generated during the lye reaction. Measure the lye separately.

Once the oils have melted, allow them to cool to 100°F. To speed up this process, you can place the saucepan in the fridge.

As the oils cool, prepare the lye solution outside while wearing protective goggles and gloves. Carefully pour the lye into the water, never the other way around, and stir continuously with a stainless-steel spoon until fully mixed.

After the lye solution is well combined, set it aside for 10 minutes to cool.

Slowly add the lye solution to the cooled oils and blend using an immersion blender until the mixture reaches a consistency similar to vanilla pudding. Be careful not to over-blend.

If desired, mix in the essential oils briefly.

Pour the mixture into a soap mold. You can cover it with wax paper and place a flat object on top if you wish, and let it sit overnight.

After 24 hours, remove the soap from the mold and cut it into 8 bars. A good method is to cut it in half, then each half into quarters, and finally into eighths.

Place the cut bars on a cookie rack and cover them with a towel for 4 weeks to allow the soap to harden and become firm enough for use.