Clear a work area on a table or countertop and cover it with newspaper or sheets of wax paper to catch drips.
For safety, make sure small children and pets are not underfoot.
Put on the goggles and gloves and weigh the lye in one small bowl (use the scale’s tare button, which allows you to measure the weight of a substance without counting the weight of the container it’s in).
Weigh the distilled water in the other bowl. In a well-ventilated area, pour the lye into the water (note: always pour lye into water, never water into lye).
Then stir the solution until the lye is completely dissolved (it tends to stick to the bottom of the container, so make sure all of the lye beads or crystals are stirred from the bottom).
Do not put your face directly above the lye solution container and avoid breathing the fumes (fumes will be strong for about 5 to 10 minutes). The lye solution will be very hot—set it aside in a safe place where no one will disturb it.
Let the lye solution cool for 1 to 2 hours until the outside of the container feels warm—about 100°.
While the lye solution is cooling, prepare the mold and the oils (no gloves or goggles needed for this part).
To prep the mold, cut 2 pieces of freezer paper so they will sit inside the shoe box, overlapping at the corners (the first piece should be slightly wider on two sides, where it will overlap about an inch or more with the second piece).
Miter and fold the corners under (like wrapping a present) so it lies flat, then cut the second piece and place it over the first piece. Smooth the paper so that it’s as flat as possible—taping it down so it stays in place when the soap is poured.
Next, weigh the coconut oil in the large bowl (again, use the tare button) and microwave for 30 seconds, or until liquid (do not overheat).
Weigh the olive oil and castor oil using the tare button, then add both to the melted coconut oil.
If using essential oils, add those to the oil mixture.
When the lye solution has cooled and the oils have been prepped, put on the goggles and gloves—because it’s time to make soap!
Carefully and slowly pour the lye solution into the oils.
Use the stick blender to mix—making sure the bell (mixing end) of the blender stays submerged in the raw soap while mixing to avoid splashing. Raw soap is caustic for a few hours even after blending, so avoid getting it on your skin.
Blend for 1 to 2 minutes, stop the blender, stir for a minute or so, then blend again—alternating between blending and stirring—until the raw soap is about the consistency of thin pudding.
This is called the “trace” stage, when drops of raw soap dripped onto the surface of the body of soap will leave a slight impression. When soap reaches the trace stage, slowly pour it into the prepared mold.
Gently tap the mold on the countertop a few times to force any small bubbles to rise to the surface.
Loosely cover the mold with a towel and set aside where it can sit undisturbed for about 24 hours.
Check the soap after 24 hours—it should be about as firm as a hard cheese.
Cut the soap into squares or rectangles with a long-bladed knife, place the soap bars into a cardboard box and allow to cure for 4 to 6 weeks.