Zany's link takes you to a great post.
But for people like me who doesn't have an oven, and who is lazy to wait til the crock pot melts everything lol, there's the microwave. Personally I think it's quicker - but not suitable for older soap unless you're ok with those bits being confetti.
Newer soap will melt easily enough even as bigger chunks with no added liquid and the resulting soap also hardens real quick. Older soap needs to be grated really small for best results. I also add a bit of oil, milk or water, stir and let it soak if it's older soap.
I heat it up in 30sec "bursts" on the lowest setting, stir in between then heat again, stir, heat and so on until it's the consistency I want.
Here's one rebatched new soap, I think only a few days old.
These are a mix. The confetti in the bottom one was thrown into the melted soap but the top soap bits were much older soap (like several months) mixed in and didn't melt. The one I mentioned a few posts earlier is the same.
Hope that helps