How to downgrade from macOS 26 Tahoe on a new Mac

An Ars Technica colleague recently bought a new M4 MacBook Air. I have essentially nothing bad to say about this hardware, except to point out that even in our current memory shortage apocalypse, Apple is still charging higher-than-market-rates for RAM and SSD upgrades. Still, most people buying this laptop will have a perfectly nice time with it.

But for this colleague, it was also their first interaction with macOS 26 Tahoe and the Liquid Glass redesign, the Mac's first major software...