The first thing to do is lay down the highlight colors. These will be the bottoms of the mold, but the top of the finished soaps. On Halloween molds, look for cat's eyes, words, ghost eyes and mouths, pumpkin's faces. Wherever there's an indentation (that will be raised on the final soap) you can put in a highlight color.
Put a few tablespoons of melted soap into a measuring cup and add the desired color, drop by drop. Mix it well. Using the disposable pipette like a combination eye dropper/paint brush, fill the indented areas (eyes, mouths, words) with the colored soap. This will work best if your soap is pretty hot. It gets thicker and is harder to paint with when it starts to cool.
