It's a simple formula of equal parts:
*I'm vegan so I use coconut but any castile works.
Shred bars of soap (I use food processor) but you can hand grate but that seems very labor intensive!
I use whatever small scoop I have to mix the 3 in equal parts. I'm not very scientific about it.
I use unscented castile so you can add essential oils if you want scented laundry.
To use, I keep a tablespoon measure by the washer and use 1 heaping tablespoon per load.