No better way to use up your leftover or expired cooking oil!! I use them for face, body, collar scrubbing, and dishes. (The oil I used in the pictures is olive oil which gives less suds but has the same cleaning power.)
[Molds] Prepare molds enough for a total of 1500ml in volumn.
[Water] Add sugar and salt to water, stir until melted.
[Lye] Put on gloves and in a well vented place, slowly stir the water with a wooden spoon and gently add lye into water (not the other way around!) Stir until fully disolved. Take extra care because the chemical reaction will produce fume and heat. Let it cool down to about your hand temperature.
[Oil] Add the Lye solution to oil (not the other way around!).
[Soap] Stir with a stick blender until the texture is like honey. (You can rest for a few minutes in between every 30 seconds of blending.).
[Molding] Pour into molds. (If the soap is too thick to handle, it maybe too cold, apply some heat and stir a little more. If the soap does not thicken, it maybe too hot, cool it down and stir a little more.).
[Cutting] If your molds or boxes are big and need cutting, cut them after a day of setting, or when it is able to keep it's form after cutting, but has not yet become too hard to cut (if you used a blender, it hardens only after a few hours).