I am looking to install Sierra from scratch in a straightforward, foolproof manner on an Air currently on Catalina. I've written 'straightforward, foolproof' because it appears that experienced persons have run into trouble doing this and I do not do sysadminey stuff to begin with.
Installing whatever O.S. you wanted used to be so easy when Macs came with DVD-Rs.
Sierra is still available as an upgrade on the App Store but as a direct install and 'visible' only to O.S.'s up to and including Sierra, so you can't directly revert to an earlier O.S. I downloaded it (on Sierra itself). The Installer says "This copy . . . is damaged, and can't be used . . ." This may be because the download was interrupted and restarted. Inside the app wrapper is 'InstallESD.dmg.'
I could re-download and if it's not 'damaged,' I could copy it all over onto an external drive to install but that external drive would have to be a bootable drive as well -- more new things to learn and more pitfalls. So is there no easier way? An "Installing Sierra for Dummies" way?