Make in iron or granite-ware pot only. I use an old waterbath canner.
Stir together:

5-1/2 pints water
4-1/2 pints grease
1/3 cups ammonia
2 tablespoons borax

Very slowly add:

1 can lye (12 ounces)
When pouring in the lye, protect your eyes. Pour close to the water to avoid big splashes. Stir with a wooden spoon 40 minutes to 1 hour, or until the soap begins to thicken. Let set overnight. Cut into bars. Can be used soon, but better if cured about three weeks.

If you have a batch that seems like it hasn't had enough grease to burn up the lye, let it cure a little longer. The "burn" will go out of it. Experience will tell you when you're making it, rub it between your finger and thumb.

To cure, stack the bars on cardboard, leaving air spaces between each bar.
For lack of any bacon grease at the time, my aunt has used town-bought lard with very good results. A very creamy, white soap is the end product.

Lye soap helps to dry up poison ivy.
Occasionally I have made a batch that seems like it would never thicken. I gave up on it, and let it set overnight anyway. It turned out okay.

Be careful to keep your lye supply rotated for freshness. An old can sat on a particle-board shelf, leaked through the container, and swelled the board upwards and downwards.