Here's the recipe I used, with step by step instructions. I found all the ingredients at my Walmart.
1 bar of soap (any kind you want, but pick something that has a nice scent)
1 cup of Borax
1 cup of washing soda
a big pot
a cheese box grater
2 empty gallon containers
And here's how the process went down. You won't make a huge mess like I did.
First, we grated a bar of soap with a box grater. If I were doing this process alone, I'd probably use my food processor to grate the soap, but what fun would that have been for the girls? :)
Then we added one gallon of water to the pot with the grated soap, and cooked it until the soap dissolved. To that, we added the borax and the washing powder and brought it to a boil. I had to rescue my son from immediate danger, but I turned around quickly when Beta yelled, "Mommy, it's a volcano!" It boils quickly!
So after it boiled, I removed it from the heat and added one gallon of cold water. I had to transfer the mixture into a larger plastic container (and at that point I spilled a bunch on the floor, you won't do that, I'm sure), and then into a pitcher so I could pour the soap into the empty milk jugs. The pitcher actually worked perfectly.
After the soap cools it gets thicker, and after washing a few loads with it, I think it works great.