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 too. 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.
So, you’re at a ritual. Think of the most ridiculous ceremony you can think of. Give it a shot – I’ll let you come up with the ridiculousness this time (it’s good practice). When you think of your mnemonic, make sure to imagine it with a Sheikh (an Arabic leader). Have the sheikh do something ridiculous to help you to think of this sheikh (しき) and then remember this on’yomi.