What you need:
Rebatching Soap Base (make it yourself or buy a premade base)
Fragrance or Essential Oil
Mold (the less detail, the better)
Colorant (liquid works best)
Step One: Grate down the soap. katwOman recommended a SaladMaster so if you have one of these, you could try that as well. The key is to have small pieces of the soap. They mix in and soften up nicely (compared to trying to melt down an entire block).
Step Two: With the water in the double boiler burbling gently, place approximately 1 Tablespoon of liquid (water, tea, beer, wine) and the grated soap in the top pan of the double boiler.
Step Three: Stirring occasionally (every few minutes) for approximately 20 minutes, wait for the soap to soften into a gelatinous mass. Once it gets to a gloppy, thick oatmeal phases and looks sort of translucent, you know you're ready. You can add more liquid but the more water or liquid you add, the softer the soap and more difficult to pop out of the molds.
Step Four: Stir in fragrance and color. I like to use approximately 1/2 ounce of fragrance or essential oil per pound of soap. Labcolors work best to color rebatch soap because they are easy to incorporate in.
Step Five: Use a large spoon and glop the soap into any plain mold and gently tap the mold on the counter to get rid of any air pockets. The more detailed the mold, the more difficult it is to get the soap to easily release. You can freeze the soap if you're in a hurry but it's best to just wait it out if you can stand it. If you don't like a more natural "back" of the soap bar, you can use saran wrap (below, right) to smooth out the backs of the soap.
That's it - you're done. You've completed a soap project in under one hour that can be totally natural and has a rustic and country chic look.