Combine:
1 cup borax
1 cup washing soda
1 cup shredded Fels Naptha soap
Add 1-2 Tbsp to laundry.
I buy the washing soda and Fels Naptha at an Ace Hardware locations. Borax is available in the laundry detergent section of nearly every big box store. I use a box grater to shred the soap. I've also tried Zote (which seemed to moist to shred well, but which you can allegedly just throw in the machine and use as your detergent in bar form), Irish Spring, Dove and a variety of other soaps. They all seemed to work somewhat adequately (no clear-cut failures).
We call the homemade laundry detergent our "utility soap" and use it mostly on towels, rags, blankets, etc.
I didn't seem to be doing a complete job removing stains and smells from our clothes, and DH likes the smell of Tide, so we use Tide for clothes, especially shirts. We seem to get a fairly good price when Costco has it on discount.
I like having the alternative/backup available and will continue to make it for the foreseeable future.