First, gather your ingredients and utensils, and arrange them attractively on your work surface.
Grate your bar soap. I chose Fels Naptha, though I understand you can use plain Ivory soap, or any other old-fashioned laundry bar soap. Be prepared to sneeze a bit from flying soap powder.
...and one cup washing soda. Light a candle for that "Little House on the Prairie" feel.
Use 1 Tablespoon powder per regular load. Use more for extra large loads or heavily soiled clothes. I understand this is a low-sudsing detergent, so you can even use it in those beautiful high-efficiency washers I so admire. Let me know if you try it and how you like it!