I have been making my own laundry detergent for three years. Here is the recipe:
1/3 of a bar of Fels Naptha soap or Zote soap
1/2 cup of Washing Soda
1/2 Cup of Borax
A big bucket (a 2 gallon size works nice)
Grate the soap and mix it with 6 cups of water in a sauce pan. Heat to boiling until the soap melts completely. Add the Washing Soda and Borax. Stir until it has dissolved. Remove from heat. Now pour 4 cups of HOT water into the clean bucket. Add the soap mixture to the bucket. Give it a good stir. Now add 22 cups of HOT water into the bucket and stir.
Let the mixture sit for about a day and it will turn into a funky gel-like substance. It's now ready to use, I use about 1/2 cup per load.
This stuff cleans our clothes just as good as anything I have bought in the store. I've also used it to pre-treat stains and it works great. The thing I like best about it is that it doesn't leave that nasty chemical/fragrance smell on clothes like so many store-bought brands do.
Borax - $3.50 for a 76 oz box at Alice.com
Washing Soda - $2.76 for a 55 oz box at Alice.com
Fels Naptha - $1.09 at my local grocery store
One batch lasts my family of 6 a little over a month (and we have TONS of laundry because of a baby and cloth diapers). One batch used to last 2-3 months before baby #4 came along. I can get about 18 batches from the box of Borax and about 14 batches from the box of Washing Soda. That's a huge savings!