1. Collect the leftover pieces.
2. Using a common potato peeler or cheese grinder, cut / grind them into small scrapes into a small bowl.
3. Add water to the bowl so that the scrapes are covered.
4. Let sit for 1 or 2 days or until you get the scrapes into soft mushy "mess"
Note: Keep in a safe place (fridge or not) where another person will not throw it out, or let others know.
You can use it like this as your laundry soap, or you can follow (optional) the next steps.
5. Blend with electric blender until it's smooth and foamy, you may add a little water if you need.
6. You can add few drops of essential oils of your choice. Keep in fridge if you don't want to use it the same day.
7. Use the blend as a laundry soap (machine or hand-wash) - it washes mine as well as a conventional liquid laundry detergent, or fill up a pump bottle and use as a hand soap. (But not good for dishes, leaves a cloudy scum, vinegar doesn't help)
I suggest trying this only with smooth soaps that do not have dried flowers, oats, bran, herbs, seeds and other visible particles in them. (I prefer smooth soap bars anyway, all the scratchy, oh, I mean exfoliating "decorations" make me crazy, dropping off during the shower ugh! For actual exfoliation I use a nylon glove when "soaping up" or some sugar / salt scrub stuff, but that's just me.)