Finely grate 1/3 bar of Fels-Naptha, about 2 ounces. Fine shreds dissolve faster and are less likely to leave visible flakes in the finished soap.

Place the grated soap in a large saucepan with about 6 cups of water. Warm over medium-low heat, stirring frequently, until every shred has dissolved; do not let the mixture boil vigorously.

Remove the saucepan from the heat. Stir in the washing soda and Borax until both powders are fully dissolved and the liquid looks evenly mixed.

Pour the hot mixture into a clean 3- to 5-gallon bucket. Add the remaining water and stir thoroughly with a long-handled spoon.

Cover the bucket and let the laundry soap stand overnight, or for 12–24 hours. It may become cloudy or thicken into a loose gel, and either result is normal.

Stir or whisk the finished soap before transferring it to labeled, covered containers. Stir or shake the container before each use if the mixture separates.