First you have to render Tallow. I do not render my own, Too time consuming. I purchase mine, using Sats.

You, obviously, have to do some calculations, as to the mold you are going to use, and much you will need for ingredients. That's another discussion.

Once you have all the precise parameters, you then melt down oil (Tallow) into liquid, you dissolve lye into water.

Depending on which method you are doing, Hot process vs cold will depend on the temps you use. Just as it sounds, you want high temp's for Hot, and cooler temps for cold. That will have to be another discussion as well.

Once you get them to the proper temps, then you blend them together until you have what's called "trace". It is thickened like when mixing a pudding, and it begins to have a "trace".

This is just basic Tallow, keep in mind. When adding colorants and scents. It becomes more difficult.