The only difference between baking soda and washing soda is water and carbon dioxide. When baking soda is heated up to high temperatures, it breaks down to become washing soda, water steam, and carbon dioxide. So, the steam and carbon dioxide are released during the cooking process, leaving you with washing soda!
The process is really simple. Just preheat your oven to approximately 390° - 400° Fahrenheit. Sprinkle some baking soda on a shallow pan, and bake it for about 30 - 60 minutes, until it changes composition. You should also stir it up occasionally, just so that it bakes more evenly.
How do you know when it changes into washing soda? Baking soda is powdery, crystallized like salt, and clumps together. Washing soda is grainy, dull and opaque, and is separate grains.
After pulling the baking sheet out of the oven and letting it cool, store the washing soda on an airtight container. The washing soda can "convert" back into baking soda if it's exposed to air.