Prepare the mold and loofah slices. Place the loofah slices into each cavity of the soap mold. Then, trim them if necessary to make sure they fit well. It's best to prepare the mold before melting the soap base to prevent the soap from cooling too quickly.
Cut the soap base into chunks. Using a sharp knife, chop the melt-and-pour soap base into small chunks to help the soap melt faster. No need to be precise—roughly cut pieces work just fine.
Melt the soap base. Place the soap chunks in a microwave-safe container, such as a large Pyrex measuring cup. Microwave in 30-second intervals, stirring between intervals, until the soap completely melts. Be careful, as the soap will be very hot.
Add the color and fragrance. If you’re using powdered colorant, mix it with rubbing alcohol to help it disperse evenly. If using a liquid color, simply add it directly to the melted soap. Stir in the fragrance oil and mix thoroughly with a spatula.
Pour the soap batter into the molds. Carefully pour the melted soap mixture over the loofah slices in the mold. It’s okay if the loofah sticks out on one side. If you want the loofah fully submerged, use a toothpick to push it down while pouring.
Allow the soap to harden. Let the soap cool and harden for at least 4 hours. For larger bars, you may need to wait up to 8 hours. Once the soap is fully set, gently remove it from the molds.