From Brooklyn Centre Wiki

If you are inclined to attempt to make your own lye soap, here are some directions:

Ingredients

2 tsp. Borax
5 1/2 pt. cold water
4 1/2 pt. melted lard
1 can lye <-- handle carefully! It is very caustic!
2 handfuls salt
2 T. ammonia

Procedure

1. Pour the water into an enamel wash tub
2. Dilute the borax in the water.
3. Melt lard and then add to the borax solution.
4. Stir in lye and stir slowly for 20 minutes until the material thickens. (WARNING! Do not pour water INTO the lye because the reaction may be too sudden and splash you with the caustic solution.)
5. Add salt and ammonia.
6. Cure overnight before cutting.
7. Place on newspaper until fully dry.

If you are really ambitious, you could try making your own lye, but we never did.