You can cook baking soda in a pot or in a stove to make washing soda. Can be done slowly at 212°F or more quickly at 400°F. Water and CO² will be driven off. Once the gasses stop, you are just drying. Stir occasionally while heating then seal once cool and dry.
Can also be done over a low fire but do not exceed 1560°F or you will be making straight soda (sodium oxide) which is much more dangerous but can add water to it to make lye for soap.
Both are fairly cheap but washing soda is 4x the price where I am.
250g in should get you around 150g out.
https://en.m.wikipedia.org/wiki/Sodium_carbonate
Not sure how much benefit there is to actually making it from baking soda yourself but it is good to know that you can. At least I think so.