How to install Sequoia on a separate volume

I do that every year. First create a new Disk on your drive using Disk Utility. Then install the current OS installer and install it to that newly created disk. Then restart your machine with that new disk as a startup disk (System Setting->General->Startup Disk).

From there, you now login and change your Software Update to accept the new Developer builds.

that's it.

There were similar questions last year when the last update came out. Here is one thread; I think there were others (but the most useful ones were on the MacRumours forums):

https://developer.apple.com/forums/thread/731398

As I recall, the answer is (1) the .ipsw is useless, (2) you can install the current macOS on your new volume and then upgrade to the beta from that, (3) there is some dodgy unofficial website that can make it easier, if you trust them. Oh and (4) if you have the same M1 Mac Mini as me and the volume you want to use is on an external USB disk, you need to connect it to the correct USB port.

Good luck!

Tried to install Sonoma on an empty volume a couple of times, and got this:

My Internet connection was good enough to download the installer. Maybe everyone is doing this and the server is overloaded?

It turns out it is possible to get a full installer for the Sequoia beta:

softwareupdate --fetch-full-installer --full-installer-version 15.0

And running that installer, I didn't get the weird error.