Fels Naptha
I don't have an ingredients list for Fels Naphtha.
One way to find such a list is from an MSDS. Just Google "Fels Naphtha" and "MSDS" and see what you get. (I do this a lot, but haven't for this.)
Do NOT use benzene in soap. Benzene is a potent carcinogen (causes cancer).
"Naphtha" is an old term for mineral spirits of one sort or another. Paint thinner, "white spirits", or Stoddard solvent, for example. Such things can have some sking toxicity, but in a soap they're probably not a a problem as the whole point is that it is to be washed away.
Do keep in mind that in use this naphtha will go down the drain, so you want something that won't damage the environment, your septic system, etc. Again, probably not a big deal, just something to keep in mind.
I do not have a procedure, but from some analogous experiments I've done, I suspect you could mix grated soap with water to form thick, heavy suds, then add the naphtha directly to that, probably anywhere from 50:50 to 90:10 naphtha to soap.