This was likely my easiest DIY project to date! I did have a little bit of trouble finding washing soda, although I didn't really go out of my way to look for it; rather, I just kept checking at stores John and I frequent, and eventually found it. Target? No. Cub? Nope. Wedge? Nah. Byerly's? Yes! Borax was easier to find, (Target, the Wedge, and Byerly's all carry it,) and bar soap -- well, all I had to do was check my bathroom for a spare bar. :)
Ingredients:
1 cup washing soda
1 cup Borax
1 bar castille soap, grated finely (I used Dr. Bronner's citrus scent)
Mix all of the ingredients together, and you're done! Eileen suggests 1 tbsp per load, although I haven't had a chance to test this out yet. I'll update when I do!