Chop up two bars of Castille soap. I use Dr. Bronner's. Chop it up as finely as you can.
Put the chopped soap in your food processor.
Put three cups of Washing Soda into the food processor with the soap and buzz it up until really well mixed and all lumps are gone.
Pour the mixture into your laundry soap container. Put three cups Borax into the food processor and buzz it up just to break down any little lumps. Pour it into the container and stir it up really well. There will be some compacted soap in the bottom of the food processor. I scrape up every bit I can and add to soap.
This is the finished laundry soap. I have a tablespoon that I keep in the container. Use one to two tablespoons per load of laundry. This is a very concentrated soap and you don't need more. Your clothes will not get any cleaner using more. If you watch your laundry like I did at first, you will see that there are no suds in the washer. It is chemicals that make all the suds in commercial laundry soap, so see the lack of suds as a really good thing!
I haven't done the math but I figure it costs about $7.00 per year.