3tbsp washing soda (not baking soda)
2tbsp liquid castile soap (I use Dr. Bronner's tea tree hemp for the antibacterial properties of the tea tree)
Combine in a 1 gallon container and add 4 cups of very very hot water. Swirl until it's all dissolved. Let cool to room temperature, then add cold water to fill the container the rest of the way.
I love this simple homemade recipe. It's no-low suds so it works great in my high efficiency washing machine and does a good job of getting the clothes really clean! It is much more "liquid-y" than traditional detergent so I use about a cup for a large load, but it's so dang cheap and simple to make that I don't mind mixing up another batch every few weeks.