This is an infinitely scalable recipe for very effective and cheap laundry detergent. It's important to do this by weight as borax and wash soda crystals are different sizes and a cup of one isn't the same weight as a cup of another. I've been using this detergent for nearly a year now and I cannot see a reason to go back to buying a manufactured brand. It works wonderfully.
The magic numbers are 3:3:5.
Three parts (by weight remember!) of borax to three parts of bar soap to 5 parts of wash soda.
That's it. Three ingredients.
Step 1: Weigh It Out
Use a scale that lets you use metric units. Metric is so much easier on the fractionally challenged. Add a zero, drop a zero. Easy.
In this Instructable I'm using the ratio to make around 550 grams of detergent. It's not critical to get precise measurements for this. The soap I used only measured in at 125 grams. Just get it in the ball park and you're good. Of course if you're scaling the recipe up you'll want to try to stay fairly close as you scale up or the ratio will get all out of whack and you'll get less than stellar soap.
I used 150g of borax, 150g of soap, and 250g of wash soda. Mix it all together.
Just as an example:
A ratio of 3:3:5
Add some zeros.
300g : 300g : 500g
Then divide by 2.
150g : 150g : 250g
Multiply and divide as needed.