Grate the entire bar of castile soap using a cheese grater.
Once the water is hot, add in your grated castile soap along with the 1 cup of washing soda.
Making sure you have a spoon long enough to mix in a large pot, mix the homemade laundry soap until you see the castile soap beginning to melt.
Allow the mixture to cool slightly before putting it in the 5 gallon bucket.
Fill the rest of the bucket up with water.
Use as you would a typical liquid laundry soap.
Notes
The castile soap will never fully melt. Your mixture might get gelatinous as it sits but this is normal. Shake your jar before pouring the homemade laundry detergent into your washing machine.