Microwaving for 30 seconds, then in 10-second intervals until the portion you’re using is fully melted. Once it’s liquid (usually after 30 seconds), add in whatever you want. Coloring, essential oils, exfoliate scrubby things, etc. We went with soap dye that we bought at the same place we bought our soap. Martha Stewart says food coloring also works. And I’m not gonna step to that, so we’ll go ahead and say she’s probably right. We also added a couple drops of lavender oil to each batch.
Once you have the color and scent you want (work quickly, it thickens fast. If that happens to you, just re-microwave for 10 seconds), pour it into the mold. If you need gray coloring, like we did, mix together a couple of drops each of blue, red, and yellow, then add more of each until you get the right carbonite-esque tone.