Here's what you'll need:
1 cup Borax
1 cup Washing Soda (NOT Baking Soda!)
1 bar Fels Naptha (I've also heard that Ivory bar soap makes a good substitute, but haven't personally tried)
1 cap full Laundry Crystals (optional/scent & 'freshness' booster)
Shred bar soap, either using a cheese grater or food processer & pour shavings into container.
Add in Borax, Washing Soda, & crystals.
Close lid & DONE. Seriousy, it's that simple. The whole process for just 1 bar takes maybe 5 minutes of my day to make! 2-3 tablespoons per load typically do the trick. ;)
Personally I quadruple the recipe (4 of everything!) & keep our powder stored in an old gallon-size ice cream tub. Between 5 peoples worth of clothes, towels, bedding, 2 pets worth of laundry, uniforms, PT clothes, etc, just ONE of these gallons last us an entire month! The only negative (if you consider this to be so) I've found is that it builds up quicker on cloth diapers than liquid detergents or cloth specific detergents, so you might have to strip them more frequently. ;)
As far as cost is concerned, it's around $1 per bar of Fels Naptha, $3/box for Washing Soda, $3.50/box Borax, & $4/bottle of Purex laundry crystals. I've had the same bottle of crystals for 3 months now & have just enough left for one more big batch. Washing Soda & Borax boxes typically last us about 2 months each.
What does all this mean? Simple...just ONE gallon of our DIY powder soap cost $8.25. That's all our laundry for a month, fresh & clean for less than $10! So much more economical that store bought detergents!