Work in a well ventilated area. Wear safety glasses, long sleeves, and long gloves. (Like the kind you wear to wash the dishes, at least some people do haha. I don't)
Whatever you're going to pour the soap into as molds...make sure you wipe them down lightly with an oiled napkin.
Melt your fats on low heat. Turn off to slowly cool to 90F.
In another stainless steel pot (yes it matters), pour your quart of water. Sprinkle in the lye slowly, stirring constantly until dissolved. Bring to 90F.
Both the fats, and the lye water must be at 90F in order for them to mix. If you need to cool the fats, you can fill your sink with water, and add ice. Place the pot of fats into it until you reach the desired temperature.
Pour the fats into the lye water mixture slowly, while mixing. This is done easiest with 2 people. We used a stick blender, and that works great, and fast. Mix until thickened, and you start "tracing" in the mixture. Meaning you can see where you've mixed a second or 2 before.
Pour into the molds.
Add any essential oils you desire. Add lots if you're doing it for scent. I did it purely for the benefits of the oils, so my doesn't smell strongly like the oils I used.
When they've started to harden, but are still slightly soft...score them with a sharp knife.
When they are completely cooled, break them apart, and lay in a cool place. Allow to cure for 6 weeks before using the soap. If you don't, it will kinda just melt away, as you didn't let it reach it's highest hardening point.