Measure out the lye and water into two separate containers, using a digital scale. Add the lye to the water (not the other way around) and stir until it's fully dissolved. It will get hot. Set it aside to cool in a safe place.
Measure out the oils and butters in separate containers. (Or at least measure the solid oils in one container, and the liquid oils in another.) Melt the solid oils, then add them to the liquid oils and melt on the stovetop on low until 140 degrees. Set aside to cool.
When the lye and the oils have both cooled down to about 110 degrees, it's time to make the soap. Prepare your mold and any fragrance you are using. Pour the lye water into the melted oil and blend with a stick blender. Blend until the soap reaches "trace", meaning that it has turned into a slightly thickened batter. There should be no visible oil droplets, and. if. you lift up your stick blender, the soap that comes off of it will rest on top of the soap instead of just sinking in. If you are adding fragrance, measure it out and stir by hand.
Immediately pour the finished soap into the mold. Cover with an old towel to insulate. it so it heats up evenly. After 24 hours, turn out and cut into bars. Allow. to cure for 2-3 weeks before using. Store in a dry, cool place.