Pre-mix the Chromium green oxide in about a Tablespoon of the olive oil. Prepare your soap mold(s) now too. The one I'm using is a clean take-out container lined with two strips of baking paper. One laid lengthwise, and the other one across. Leaving overlapping paper will help you get the soap out when it's ready.
Next, dissolve the lye (Sodium hydroxide) crystals in water. Gear up with eye protection, gloves, and wear a long sleeved top. In an airy place, outdoors is best, pour the lye crystals into the water and stir well. There will be a lot of heat and steam so be careful. Try not to breath it in. Leave outside in a safe place, or in a shallow basin of water to cool.
Melt the solid oils in a stainless steel pan on very low heat. When melted, remove from the heat and set on a pot holder. Pour in the liquid oils including the colored oil.
Measure the temperatures of the lye-water and the oils. You should aim to cool them both to be about 120°F / 49°C.
Pour the lye-solution into the pan of oils. I tend to always pour the liquid through a sieve to catch any potential undissolved lye or bits.
Dip your immersion blender into the pan and with it turned off, stir the mixture. Next, bring it to the center of the pan and with both your hands, hold it on the bottom of the pan and blitz it for just a couple seconds. Turn it off and stir the soap batter, using the blender as a spoon. Repeat until the mixture thickens up to 'Trace'. This is when the batter leaves a distinguishable trail on the surface. The consistency will be like thin custard.
With your spatula, stir in the essential oils. If you'd like a few calendula petals on the inside of the soap, stir in the optional 1/2 tsp of them now. Working quickly, pour the soap into the mold(s). Sprinkle the top with dried peppermint first then a few more calendula petals.
Turn your oven on to very low and heat for just a minute or two until it's 100°F / 38°C. Then turn your oven off, and pop your soap mold(s) inside. Leave overnight. Oven-processing the soap like this intensifies the color.
The next day, take the soap out of the oven and set someplace to rest for another day. Once 48 hours have passed, you can take the soap out of the mold(s). Cure it for 28 days before using. Curing means leaving the bars spaced out on a protected surface out of direct sunlight and in an airy place. This allows the extra water content to fully evaporate out.
Once made, your soap will have a shelf-life of up to two years. Check the oil bottles that you're using though -- the closest best-by date is the best-by date of your soap.