My son was complaining that the bathroom was out of hand soap. So instead of going out to buy some, I told him let's make some.
We took 1/3 of a bar of Fels Naptha soap and grated it really small. Then I put it in a stainless steel pot, added 1 cup or so of water, and heated it up. I kept stirring it to keep it from burning. It became really foamy, but once all the flakes were melted I took it off heat to let it cool. Once it cooled a little i poured it into the squirt jar I had from the last store bought batch.
Well, a few hours later it has solidified into a semi-solid mass, too thick to use. Out comes the pot again, I used a knife and a little water to get the solidified soap out of the bottle, added 2 more cups of water, and stirred it again. Once it was all mixed and foamy again, I poured it back into the squirt bottle (and a 2nd jar to be recycled because now there is approximately a quart of soap).
Works quite well. Has a nice mellow smell.