In a medium cooking pot start 4 cups of water to a on high heat.
Measure out the 1 cup of borax and 1 cup of washing soda, mix to combine.
Grate your Fels Naptha laundry bar. I used a hand cheese grater, but you can use a food processor or even a knife to shave off the pieces.
If the pot is boiling, reduce the heat to medium and add in your grated Fels Naptha bar. You will want to stir this continuously for around 15 minutes until the bar if fully melted and incorporated into the water. You DON'T want to let the pot over boil so if the mixture starts boiling heavy and rising to the top of the pan remove it right away. It will be the biggest mess of your life if you let that boil over. Not kidding. Just keep it at either almost boiling or barely boiling the entire time and keep stirring.
After the first 5 minutes you should only have a few big pieces left to dissolve. But for some reason they take forever to dissolve. Keep stirring until its all dissolved (about 15 minutes).
Then add in your Borax and Washing Soda. Mix for around 5 minutes until they are totally dissolved as well. I can tell when it's fully dissolved because there are no more grains at the bottom when I stir.
Then add your mix to a large bowl. Add in 4 cups of warm water and stir to combine.
Allow this mix to sit for 6 hours.
After the 6 hours is up it will look a little jelly-ish. Take a wisk and smooth the separated water and jelly until it's as smooth as you can get it. Store this concentrated mix in a large sealed container or jar.
When your old bottle of liquid laundry soap runs out add in one cup of this gooey concentrated mix plus 7-8 cups of water. Shake it up well until it's nice, smooth and incorporated together.
I use the cap that came with the old liquid laundry around 3/4 full so around 1/3 cup of liquid detergent per load of laundry. If you have a High Effenciency machine you will probably want to do a little less than that.