I have read, more than once, that people used to save their grease left over from cooking and then use that to make soap. I've been making soap for about 33 years and until last week, I never tried to do that! I kept a container in the freezer, and every time I had some grease (chicken, pork, or whatever) I put it in the container. On soap making day, I had accumulated 75.65 ounces of grease. Well, that's how much was left after I purified it. I put it all in a big kettle, added maybe a quart of water and boiled it so the impurities would settle to the bottom. Then I took the floating resultant fat off the top and weighed it.
Next, I went to the MMS lye calculator and plugged in that value. I listed the fat as "lard" because I had no idea how much of each kind of fat I had. The blessed calculator then tells you how much soft water and how much lye to use, and the procedure to follow. My intention here is not to give you a soap-making lesson, but rather to encourage you to try this. Here is a picture of one of the bars of soap. I just washed my hands with it, and it is very nice and doesn't smell funny or anything! It bubbles nicely and doesn't seem to be drying. I am very pleased.
Here you are looking down into the cardboard box where I am letting the soap bars cure/dry. Instructions usually suggest you wait 6 weeks before using soap. It depends on the soap. This does not seem to have any "free" lye in it. It did not sting my hands. But of course, be careful!
If you feel intimidated by soap making, please don't. There are some lovely how-to videos on www.youtube.com, that will be very helpful. Once you understand the process, which is NOT complicated, and the safety precautions to take, it is very easy and has become one of those things I can do if I know I'm going to be in the kitchen anyway, and so I make it while I'm cooking other things. Many posts ago I mentioned doing things "in my sleep." Within a few batches, you'll be making soap in your sleep too!
Just last week, my daughter and I made a batch of facial soap for her. It is 1/2 coconut oil and 1/2 olive oil. Very nice soap!
I buy my lye through eBay.
The reason I started making soap all those years ago was because money was tight, and our children seemed to delight in letting bars of soap disintegrate in the bath water. That was more expense than I could handle, and I felt that if I made my own, and it is SO much cheaper, then that would help the budget. I was right. :)