Cut the soap base into cubes that are around 1 inch in size.
Place into a microwave safe measuring cup. Heat in the microwave for 30 seconds, remove and stir, then heat for additional 30 second increments, until fully melted, stirring well between each heating session. This can also be done in a double boiler.
Once completely melted, add the tallow and mix through until completely melted.
Let it cool for a few minutes, then add the essential oils, if using and keep stirring.
Pour into the soap mould and let sit until hardened.