Well we have the bottles, and two bars of very dry, very old fairy household soap
so an old jug, a sharp knife, and some warm water later we now have hand gel wash

I thought I might have to use the grater but the soap was dry, and a knife shredded it easily (like scraping a carrot) not a clue how much water to add so when the jug was about a third full I added the same of warm water, stirred it and left on the windowsill in the warm (the jug has a lid) looked in a week later and it was a very thick gel so added more water and left it another week
Today I decanted some into the handwash bottle and voila

