Safe and Sure Way to Install Sierra / Ongoing Regressions in OS X 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?
This is an overview. I'm going to assume you know your way around a Mac enough to fill in the blanks.

You should be able to download Sierra from the App store. (or here: https://support.apple.com/en-us/HT208202) Note that if you are getting an error message about a damaged installer it may be that the installer is old and the certificate in it is expired. I have heard that setting the clock back a number of years will also fix this issue.

Then using the command Apple recommends using (at least 8GB) flash drive create your bootable installer. 

sudo /Applications/Install\ macOS\ Sierra.app/Contents/Resources/createinstallmedia --volume /Volumes/Untitled --applicationpath /Applications/Install\ macOS\ Sierra.app


Or you could use something like DiskmakerX you can create a bootable macOS Sierra install disk a different way. I use this method as I like me some GUI apps.

Once the bootable installer has been created on the flash drive and with the flash drive plugged into your Mac (and a backup of your Mac somewhere safe and disconnected). Boot the Mac holding the ⌥ Option key.

When you get to the Startup Manager screen select the Sierra installer and let it boot. It may take a while. 

Eventually you will be asked to select a language and be given a window with several options.

Instead go to the Utilities menu > disk Utility and reformat the internal drive. This will, of course, remove everything from the drive so do be very sure about your backup.

When that is done quit disk utility and select the option to install Mac OS Sierra on the drive you just formatted.

Now, all of this assumes that macOS updates that you have previously installed have not updated the firmware on your Mac to the point that macOS Sierra won't work. That is possible but I don't know how likely it is. So proceed at your own risk, and have a macOS Catalina bootable installer standing by, just in case.