There are a bunch of recipes out there for laundry soap, you can try out different ones, if you like. Here is the one that I use and love:
1/3 bar of Fels Naptha soap, grated
1/2 c Arm & Hammer Washing Soda (not to be confused with Baking Soda!)
1/2 c 20 Mule Team Borax
I get all of my ingredients at WinCo. This probably costs around $7 for these three items, and just think of how many batches you can make out of each box! Fels Naptha runs about $1 and I use a 1/3 of the bar for each batch. I grate up the entire bar and then put it in three separate ziplock bags, so I don't have to grate it up each time I make some.
-First, in a large pot on the stove, heat up 6 cups of water and pour in your 1/3 bar of grated Fels Naptha soap. Stir until dissolved (just on medium heat. . . don't bring to a boil).
-Then add in the washing soda and borax and stir until dissolved.
-Add 4 more cups of water.
Now what you do next will depend on how you are going to store your soap. This recipe makes 2 gallons. I have one of the old Costco laundry soap containers that this fits perfectly into. But, if I didn't, this is what I would do:
-Stir or shake until it is all mixed up. (At this point, if you are not using a laundry soap container, pour your mixture into whatever containers you will be using to dispense the soap. You can pour it into two different containers if you don't have one big enough).