1 cup borax.
1 cup baking soda.
I bought the Sunlight pure soap so I would have that "lemony laundry" scent. But you can use whatever you want. Or you could be extemely crafty and use home-made soap ;)
Basically all I did was grab my grater (you could also use a food processor) and grate away until the soap was all finely grated. I figured I should try to grate as fine as possible to avoid any chunks not dissolving in the wash.