There are a lot of different recipes out there to try, but I liked this one because it's only 3 ingredients, and is very simple to put together.
1 bar of Fels Naptha - $.97
1 cup Borax - $2.98/76oz
1 cup Washing Soda - $3.24/55oz
I think you can probably find all of these things in the laundry aisle of your grocery store. I found them in my Wal*Mart Supercenter. I'm sure the prices vary by region, but that is what I paid. For me it came out to about $1.86/42 loads.
Grate the Fels Naptha bar. Mix in the Borax and Washing Soda. Run it through a good food processor to turn the mixture into a really fine powder. Use 1 Tablespoon per load. Maybe add a little more if the load is extra soiled. Voila!