2.5 cups Baking Soda
2.5 cups Washing Soda
2.5 cups Bar soap (grated)
I chopped up a bar of Ivory soap, a tiny bar of hotel soap we had laying around, and two bars of my Castille Soap. Then I ran them all through the food processor (which is very small, so I did one bar at a time). The Ivory is a very dry, crumbly soap and turns to powder easily. My homemade Castille soap is wetter and more flexible and doesn't powder when food processed. I discovered that if I added some of the dry ingredients to the food processor with the soap, it then chopped up into tiny little crumbles pretty easily.
You use 1/8 cup of powder per full load. This means that with my tiny little apartment washing machine, I should probably use about a tablespoon. At that rate, this batch of detergent should last me till next year. I also have to burn through the rest of the "liquid" detergent and then a bottle of store-bought detergent that my dad gave me that didn't work well in his new HE washer.