…one of those white slips
Handed her cup and piped, the pretty one,
“Drink, drink, Sir Fool,” and thereupon I drank,
Spat—pish—the cup was gold, the draught was mud.Alfred, Lord Tennyson, The Last Tournament
You can build a system and then build its reference sheets, or you can build the reference sheets first.
The latter is a backwards way of going about things, but it’s what I’m doing. Not that I have no idea what the game will be before setting up each field or box on the sheet - there’s an iterative process - but I make a lot of judgments based on what can fit well on the pages.
And there’s some advantages. I can get a lot of inspiration poking around with Affinity and trying things out, and can avoid running into systems that would end up making difficult sheets - like ones that require a lot of erasing. Certainly there’s some benefit to having put a little thought into the UX before trying to make your cool systems work.
But I don’t think I’d advise people do this in general. For me, it’s convenient, and compelling, and fun. But there’s a lot of other ways to do this.