Ritual, Ceremony, Equation
To construct a whole ceremony, you can’t just make it up on the spot. That is not a ceremony. A ceremony has a past. A ceremony is done a certain way because that’s the way it’s always been done. In short, to build a ceremony, you have to create a ritual first. Then, the ritual can be constructed into a bigger, more lavish ceremony. This kanji also means equation for some reason. Just think of a ceremony that involves doing math and coming up with equations. How strange would that be? Pretend you're at this ceremony.
Ritual and ceremony kind of mean the same thing. Don't worry too much if you mix them up a little - each will help you to remember the other. There's a reason this kanji is made up of the ceremony radical already!
しき
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So, you're at a ritual, but not just any old ritual. This is a full-blown ceremony and it's incredibly chic (しき). Imagine just how chic it is. Give it a shot – I'll let you come up with the chicness this time (it's good practice).
It's good to practice making your own mnemonics up every once in a while. It'll help you to add on to the mnemonics already here and make better ones more personal to yourself!