Here is a nice tutorial from Brambleberry on making a lavender rebatch soap.
Charming Rustic Soap
Hemp Rebatch Base
Lavender 40/42 Essential Oil
Dark Purple Labcolor
About 1 Tablespoon liquid
(Note: Rebatch soap is unscented, uncolored Cold Process soap. With this simple starter base, you can customize your soap by adding fragrance, essential oil, liquid color, mica, or herbs. Once you’ve molded your soap, you have a unique, handmade soap creation.)
There are many different ways to make rebatch soap. Below we explain the double boiler method:
Step 1: Grate up the soap base and place in the top of a double boiler. Stir the soap while it melts and keep the water and at a gentle simmer.
Step 2: You’re looking for the consistency of oatmeal. Add some liquid to the soap if needed. Typically, you just need to add a couple of tablespoons per pound of soap – but sometimes, if the soap base is older or really hard, you may need more.
Step 3: Add the essential oil and purple Labcolor and mix well. We suggest 1/2 ounce of essential oil per pound of soap. Sprinkle in some dried lavender herbs and stir gently.
Step 4: Place soap into mold – glopping is the best technique. Immediately get the air bubbles out by tapping (or banging) the mold onto the surface. You are trying to get the soap to settle down into all of the details of the mold.
Step 5: Allow the soap to set-up in the molds for a few days or until the soap is firm enough to remove. The soap may need an extra week or two to fully harden-up depending on how much liquid you added.