All you need is 1 bar of Fels Naptha, 1 cup of Arm & Hammer Washing Soda, and 1 cup Borax. That's it.
All you have to do besides measure the powders is grate the bar of soap.
It fits perfectly in a quart jar.
Make sure to label your soap and keep a tablespoon with it!
You can just keep the supplies around to use in the case of an emergency or you can make this all the time. There are 64 tablespoons in a quart so this should last you for about 64 loads!