Start by grating the bar of soap. Then melt the grated soap in 1 gallon of water on medium low heat. Do not boil and stir often. When the soap has melted add 1 cup of borax and 1 cup of washing soda and stir until dissolved. Pour your soap mixture into a big bucket (something that holds more than 3 gallons) and add 2 more gallons of water. Now add an essential oil of your choice if you would like and stir well. Let it set over night. It will gel up and separate over night so stir and stir or use an immersion blender to make it nice and smooth.
Use ½ a cup per load of laundry.
Recipe by Queen Of The Red Double Wide at http://queenofthereddoublewide.com/homemade-liquid-laundry-detergent/