A couple of years ago me and my sister were cleaning out my grandmas house and came across over 100 bars of Fels-Naptha. We were not sure what to do with it but we hated to just throw it out. She had a friend who was making their own laundry soap and it had Fels-Naptha in the recipe. This is an easy recipe and it makes good soap with no dyes or perfumes. It also makes me feel better about not using those huge plastic containers that the soap usually comes in. I make it in liquid form in a 5 gallon bucket, I use one cup for a load of clothes but you can experiment to see how much you want to use. It does NOT sud up so you might think you need to add more, but it will never make suds. You can add essential oils if you want it to smell good but I don't.
2 bars fels-naptha, chopped or grated
2 cups borax
2 cups arm and hammer washing soda (not baking soda)
You can find the borax and soda in the laundry aisle.
Dissolve the grated fels-naptha in about 6-8 cups water. I cook it on the stove on low and use a whisk to help it dissolve faster.
Boil enough water to fill your 5 gallon bucket about half way.
Add borax and washing soda powder to bucket, add boiling water and dissovled fels-naptha. Stir well. At this point I like to move the bucket to the laundry room before it is too heavy and then add enough hot water to fill to the top. Let it cool off, the next day it will gel up, that is normal.