There are directions all over the internet forit. Essentially you need lye, which is obtainable by soaking wood ashes (as from your fireplace) in water, and some form of fat. Some heat, some stirring and you have the stuff.
This is not something to be though of when you run out. Soap needs to be cured for 4-6 weeks before using. The process of leaching lye from ashes is a little difficult and can't be done while you are waiting for a bath. (Well it could but until the lye leached and the soap cured, I am not sure you would have a lot of friends left;-))
We always keep at least a 6 month supply of soap on hand, and supplies to make at least 3 batches.
I have found that super-fatting (adding at trace) with mineral oil works very well for a fraction of the cost of most oils that are commmonly used.