Prepare the mold and loofah slices. Place the loofah pieces into each cavity of the soap mold, cutting into smaller pieces if necessary. It's best to do this before preparing the soap so that it doesn't begin to cool as you get everything ready.
Cut the soap base into chunks. Using a sharp knive, chop up. the melt and pour base. This is just to make the melting process faster, so there's no need to be precise.
Melt the soap base. Place the soap pieces into a microwave safe container such as a large Pyrex measuring cup. Microwave in 30 second intervals until it is completely melted. (The soap will be very hot.)
Add the color and fragrance. If you are using powdered colorant such as mica, mix it into the rubbing alcohol to help it disperse better. (If using liquid color, just add directly to the melted soap.). Mix in the fragrance and stir well with a spatula.
Pour the batter into the molds. Pour the soap batter over the loofah sponges. If they stick out a bit on one side, that's okay. You can use a toothpick to hold down the loofah as you pour if you want it fully covered.
Allow to harden at least 4 hours. Let the soap cool for 4-8 hours, then remove from the molds.