Chop the bar of soap into small chunks with a sharp knife and then whizz in a small food processor for a few seconds until it's a fine, crumbly powder. (I use the little food processor that attaches to my whizz stick). It's pure soap, so it's fine to do this in kitchenware!
Put the soap crumbs into an airtight container.
Next, put 2 cups of washing soda crystals into your food processor and blitz until they are fine crumbles too. Add them to the soap crumbs and stir together well so it's all combined. Rinse food processor and utensils well to remove any trace of soap or washing soda.
I use 2Tbsp for a full load in my top loader. Add ¼ cup white vinegar to the fabric softener compartment of your washing machine, and you can also add a few drops of lemon essential oil for fragrance (and grease-busting power - apparently) to the vinegar.