Put the grated castile soap pieces in a food processor bowl. Pulse until only large crumbs (no chunks) remain.
Add remaining ingredients to the food processor and alternate pulsing and blending until well combined.
Store detergent in an air-tight container to help avoid clumping.
Simply add about 2 tablespoons directly into your washer bin with your clothes (you might want to only use 1 tablespoon for HE front loaders. Add more, if desired, for larger or more heavily stained loads.
You can also simply chop up the castile soap bar instead of grating it, but grating will help the food processor work better without the concern of damaging the blades.If you live in a humid climate, your laundry detergent can have a tendency to clump up. To help avoid that, you might wish to keep a dessicant in with your detergent.