1. Using a box grater, finely grate bar of soap. (Alternatively, cut soap into chunks and pulse in a food processor fitted with an S blade.)
2. To a large bowl, add grated soap and washing soda; stir well to combine.
3. Store in a glass jar with a lid.
4. To use in traditional washing machines, add ¼ cup to regular loads or 1/3 cup to large or heavily soiled loads. For HE machines, add 3 tbsp to regular loads or ¼ cup to large or heavily soiled loads. (*NOTE: If using, add 20 drops of lavender oil and 10 drops of lemon oil to soap before running machine. Use less oil if washing a smaller load.)