Grate the soap into powder and put it in a pot with water. Heat and stir constantly until the soap is dissolved into the water.
Add the soap solution, the washing soda, the baking soda, the borax, and any essential oils you want to use, to the bucket and stir.
Add more hot water, as you continue to stir the ingredients, until they are all dissolved.
Cover the bucket and let the detergent cool and gel overnight.
Store the detergent in a container with a lid.
Notes
It is normal for this detergent to be a bit thicker than store-bought laundry detergents, so don't worry if yours looks a bit gloppy. Just stir it up again before using it.