This is the recipe I used:
1 bar Fels Naptha bar soap (about 4 cups grated)
2 cups 20 Mule Team Borax
2 cups washing soda
Grate the soap. This can obviously be done my hand but my parents gave me a fancy KitchenAid grating/slicing attachment for Christmas (thanks Mom and Dad!) and it made quick work of the grating.
Mix the grated soap with the borax and washing soda. Store in an air tight container. Use 2 tablespoons per load.
(I love how the grated soap looks like lemon zest. Makes me want to make lemon bars.)
I also used 1/4 cup white vinegar in the rinse cycle in place of fabric softener (I usually use dryer sheets). The clothes didn't smell like vinegar but I think I might try adding lavender essential oils to the vinegar.
My next way to save money is to stop using shampoo. Sounds gross, I know. Read this and you'll want to try it too!
***update: see this post for a new laundry detergent making tip!