Grate the Fels-Naptha soap. Add the soap shreds to a pot with 4 cups of hot water. Put the heat on medium-high, and stir until completely melted - about 10 minutes.

Fill up a 5 gallon bucket halfway with hot water (I like to use the shower for this, it's easier in my opinion). Pour in the melted soap mixture and give it a quick stir.

Stir in the borax and washing soda. If your'e using essential oil, now is the time to add it. You add in ½ oz to 1 oz, depending on how scented you want it. Although, I never really measure it anymore - I just shake in enough drops to where I feel satisfied. Stir the contents again.

Fill up the bucket with warm water, all the way to the top - and stir again. If you don't have a utensil long enough to reach the bottom, just put on a pair of rubber dish gloves beforehand.

Snap on the lid, and let it sit for 24 hours to gel up. It will have the consistency of egg noodle soup - watery with glops of gel. This is what it should look like! If you would like a smoother consistency like the store-bought brands, you can use an immersion blender at this point to smooth it out. I don't bother with this step though personally - the 'glops' dissolve just fine in the wash - and having a smoother consistency has no effect on the cleaning power or finished product.

At this point, you can divide it up between different containers like empty laundry detergent containers, cleaned out milk jugs, etc. I prefer to use 1-2 empty containers, and leave the rest in the bucket to save on space - refilling my containers as necessary.