I got this recipe somewhere on the internet many years ago and have been using since then.
Make in a 5 Gallon bucket you can buy at Home Depot or Lowes.
1 or 2 large bar(s) of soap (I use Fels Napha)
1 cup Borax
1 cup of Washing Soda (Sodium carbonate)
Grate the soap and place in a double boiler. Cover the grated soap with
hot water and heat until the soap is melted into the water. This could take
some time, so be patient.
When the soap is melted, put about one gallon of very hot water into the
5 gallon bucket. Stir in the melted soap, the borax and the washing soda.
Stir until mixed thoroughly. Continue adding hot water to the mixture until
you have approximately 5 gallons. Cover and the Laundry Soap is ready to
use. As it sits overnight it will gel up. Use about 1 – 2 cups per wash load.