Handmilling is a fun way to vamp up your soap. Handmilling involves grating and melting down some week-old handmade lye soap in a microwave, doubleboiler, crockpot, or in a double-bagged ziploc dropped directly in gently boiling water. Whichever method you choose, the goal is to melt down your soap until it is runny or mushy. Be sure that after you grate your soap (use a cheese grater) that you add a little liquid to help it melt and avoid drying out. You can add water, milk, oil, an herbal infusion, fruit juice, or anything of that nature. Handmilling is far less "precise" practice than cold processing. As with cooking, in handmilling you just add a punch of this or that until the mixture looks about right. For one pound of shredded soap, I usually add about one cup of liquid. If you aren't sure, just add a little liquid as you go until it has the consistency that seems right to you.
Microwaving, while convenient, is probably the worst way to melt down your cold process soap because it doesn't thoroughly and uniformly heat the soap. For example, some parts of the soap can get dry and crusty while other parts are just starting to melt. However, some soapmakers swear by the microwave method. Be sure to microwave your soap in short bursts to avoid overcooking it. It doesn't take much for the soap to expand and spill out of the container.
Melting down your soap shreds in a crockpot slowly over the course of a few hours is supposed to produce the finest quality handmilled soap. The double boiler method involves boiling water in a large stock pot with your soap shreds sitting in a bowl on top of the pot. The steam heats and melts the soap in about 30-60 minutes. Don't let the pot boil dry. Probably the most popular and relatively simple way to handmill your soap is with double-bagged ziplocs in boiling water. With this method, place your soap shreds and liquid in a ziploc and place that into another ziploc. Place this in a pot of gently boiling water for about 30 minutes until it is soft or liquidy. The ziplocs will feel like they're getting thin in the hot water, but they will survive the extreme temperature just fine. The bags may also fill with steam as they boil—this is normal.
After you've melted your soap, mix in your additives (colorants, scents, dried herbs, etc) and mix well. How much scent or colorant should you add? Whatever looks right to you. Be sure to sniff the soap and make sure it smells a bit on the strong side as the scent will fade when the soap cures. If you are using the popular ziploc bag method, put your additives directly into the bag, seal it, and knead the bag. After mixing in your additives, cut a bottom corner off the ziploc and squeeze the handmilled soap out of the cut into the mold. Tap the mold several times on your countertop to help pack it down and remove air bubbles. Cover your molds with saran wrap and cure as usual.