1 bar soap (ZOTE, Fels-Naptha, Ivory)
1 cup of washing soda
1 cup of borax
You can use a hand grater for the bar soap, but I feel like I am tasting it when I do that, which is very unpleasant, so I started putting mine in a food processor that is now devoted to these kinds of projects. I usually make big batches so I don't have to make it it very often. I'm always surprised at how far one big batch lasts.
I can find ZOTE, Fels-Naptha and Ivory at the local grocery store. I like to use Ivory and either the ZOTE or the Fels-Naptha because the latter two are specifically intended for laundry use. There are other natural soaps out there, but these are the only ones I've used and can vouch for.
This laundry detergent is septic safe and works for HE washing machine. 
