Grate the entire bar of Fels-Naptha soap. (A coarse grater will do the job and take less time.)
Heat in a large saucepan over medium-high heat with 8 to10 c of water until soap melts.
Add the Borax and washing soda and stir until dissolved.
Pour a gallon of hot water into your bucket and add the heated mixture. Stir well.
Add enough water to make about 3 gallons (about ¾ full in a 5-gallon bucket) and stir again.
Let sit overnight and the mixture will solidify to a watery gel. Use a ¼ c per load, more for heavily soiled loads. Be sure to switch the soap compartment to the "liquid detergent" setting if your machine has that option available. If you want to infuse a particular scent in your laundry soap, add a few drops of fragrance or essential oil to the detergent after you've put the ¼ c detergent in the laundry machine compartment. This is not as effective or economical as scenting the fabric softener alone, but the laundry room will smell wonderful.
For a really tough, dirty load, add ¼ c Borax and ¼ c washing soda in the "auto soak / pre-wash" compartment. That will take care of just about everything. Be sure to press the "auto soak" button on the front loaders instead of "pre-wash." Auto-soak is more effective in removing stains than the pre-wash cycle.