Older bars are GREAT for this recipe because it requires very little effort to powder them. If it's fresh fels, you can use this method to powder it quickly. If it's an older bar, you can follow those instructions for making powder, but SKIPPING the microwave altogether and just cutting the bar into small chunks and then pulsing. Here is an example of the texture you can expect from an older bar of Fels:
1 Bar of Fels Naptha, Powdered
1 1/4 cups Arm & Hammer Washing Soda (not homemade)
1 1/4 c. Borax
Combine the ingredients until they are well blended into a fine powder in a medium size bowl.
Add 1/2- 3/4 cup HOT Water to the bowl of ingredients:
Then immediately mix well until the mixture resembles dampened corn meal.
Next, Using a Tablespoon, scoop the mixture into the rounded measuring spoon, packing it in tightly as though you were measuring brown sugar. Turn it onto a hard surface and lightly tap once to get the mixture to drop out of the spoon onto a piece of wax paper or parchment paper. Repeat until all the mixture is gone, about 40 Laundry Tablets.
If you'd prefer, you can pack them into (small cube) ice cube trays instead (like these star shaped ice cube trays I got at the Dollar store).
Leave them overnight to dry completely. Store in a Covered container. To use, add 1 tablet per load of laundry. For Front Loaders or HE Machines, add the tablet directly with the clothing.
1 Bar Fels .99
Borax 76 oz box = 9.5 cups $3.29 box = .34 c per cup - 1 1/4 cups = .42¢
Washing Soda 55 oz box =6.875 cups - $2.97 box = .43 per cup = 1 1/4 cups = .54¢
Total Cost $1.95 or .05¢ per load