I collect the little slivers until there are a dozen or so of them. Periodically I grind up the slivers with a salad shooter or just grate them with a grater. Then I cut the bottom off an oval plastic bottle to use as a soap mold. Take the grated pieces and put them in the bottle bottom. Add some water to the mix. Then put it in the microwave --- about 10 to 15 seconds at a time. The soap will begin to bubble and rise. Take it out and tamp it down and replace in the microwave and repeat for another 10 seconds. Do this a few times till the soap will tamp down and the air bubbles are removed (you may need to add a bit more water to get the proper mix). Then put it all aside to dry out (about a week or so). When it's dry, take it out of the bottle bottom, and let it dry out another week or so. Then, use it.
I had about 30 bars of hotel soap collected over the years. I ground them all and made 5 larger bars of soap I'm currently using in the shower every day. Works great.