... I saw one of these on eBay and wondered how the heck to use a cube of soap.
I cut off a thick slice and paste it to the bottom of a lather bowl or shaving mug. Works great, as does the soap -- I just used it again this morning.
You could always rub it on like a shave stick.
Mine looks like this after about five days. Another slice can easily be stuck on before it's gone.