Here's what you need:
Some glycerin (available at any craft store), a microwaveable container, some herbs, and a mold of some sort (could really be anything--I used a square cardboard container).
So, first you microwave the glycerin until it melts completely (microwave about 30 seconds at a time). Meanwhile, chop the herbs.
Once the glycerin is melted, stire in the herbs.
Pour into your mold of choice.
Let it cool completely, then remove the soap from the container.
Now you're left with one big block of soap and you're free to cut it up as you like.
I chose to simply slice mine into three large slices.