I have tried a couple different recipes and I like this one the best:
Home Made Liquid Laundry Detergent
1 bar grated Fels Naptha soap
enough water to cover grated soap
5 gallon bucket
thick dowel rod for stirring
1 cup Arm & Hammer Washing Soda (not baking soda)
1 cup 20 Mule Team Borax
4 1/2 gallons hot water
Put grated soap in a saucepan with enough water to cover the soap. Heat on low heat, stirring occasionally, until dissolved.
Fill 5 gallon bucket with 4 1/2 gallons hot water. Add melted soap. Stir to combine.
Add washing soda and borax; mix well.
As it cools, the mixture will thicken. If it thickens too much, add a little hot water and mix well.
Use 1/2 to 1 cup per load of laundry.
Update: I have started using 2 cups of washing soda and 2 cups of borax when I make mine because my kids are dirty. You will still need to pretreat stains, especially greasy ones.