My recipe for homemade powdered laundry soap:
1 part Washing Suds
1 part Borax
1 bar of soap of your choice
You're going to shred the bar of soap using a cheese grater. I find the smaller shreds work better, as they dissolve more easily in the wash.
I prefer to measure in cups. 1 cup Washing Suds, 1 cup Borax, 1 bar of shredded soap.
Mix together well. Add to a container with a secure lid. It only takes 1-2 tablespoons per load. Because I had kids, dogs, and cats, I always used two.
When you first use it, it won't seem like that's all you need, but it is.
You can scale this recipe to as large or as small as you want. I always made three batches at once. It lasts for several months.