All you need is baking soda,a pan, and an oven set at 400 degrees
Pour your baking soda in a pan.
put into a 400 degree oven.
It will take about an hour to an hour and a half.
When you make it you will be able to see the difference. Washing soda looks and feels different then baking soda. Washing Soda is grainy, dull , opaque and is separate grains where as baking soda is powdery and crystallized and reflects light and clumps together.
The washing soda you make will look a little different then what you buy. When I first started doing this I had to put some baking soda on a plate and put the homemade washing soda next to it so I could tell the difference but after you do it you will be able to see the difference.
I put baking soda on the top left and store bought washing soda on the top right and the homemade in the middle it is kind of hard to see in the picture but there is a difference.