How do I melt used bars of soap?

Add some water and melt on stove, you may have to mix or blend if there are bits that don't melt properly. Medium low heat

Here's a good video tutorial: https://www.youtube.com/watch?v=0m69EiNy77s

I never have any, I simply let the smaller piece sit in a wet environment, and overnight it softens, then press into the new bar after letting it get soft from sitting in the wet spot at the same time, so wet to wet and it will bond. Never any small pieces left. 88 years old and been doing it a LONG time.

I made a self soaping washcloth. Envelope style.

My husband and I just smoosh small slivers of soap together to make bigger bars.