You need 3 pounds of tallow and 12 ozs of lye (sodium hydroxide).
Method:
Pour 3 cups of very cold water into the 2-quart plastic saucepan.
To the cold water slowly and carefully add the lye, a little bit at a time, stirring it with the a wooden or plastic utensil. (Use plastic gloves for this; test them for holes first.) Do not breathe the vapor or lean over the container or have children nearby. Above all use no metal. The mixture will get very hot.
Let the mixture cool at least one hour in a safe place. Meanwhile, the tallow should be warming up to room temperature in a plastic dishpan.
Slowly and carefully, pour the lye solution into the dishpan with the lard.
The lard will melt so mix thoroughly for at least 15 minutes, until it looks like thick pudding.
Leave it to set overnight, then, next morning cut it into bars of soap.