Grate the bar of soap. I use the smallest side of my 4-sided grater.
Put in plastic bag or other container with the washing soda and borax. Mix well. Note: I have very hard water so I use more soda.
Dissolve in a small container of warm or hot water before adding to a cold water wash. You can put it right on a warm or hot wash.
For a load of not-too-dirty laundry (clothes I've only worn for office work), I use 1/4 cup.
If I've been out working in the mud and dirt and dust, then sometimes I need a whole cup, and maybe a pre-soak. And I mix in more borax.
You can pre-treat stains with more soap right off the bar.