Before attempting to download and install macOS Sierra 10.12, you should read carefully through my suggested preparations. It is breaking a lot of old apps. If you have not updated an app to its latest version, then you may find it impossible to update through that app once you have installed Sierra. It is much easier to fix this before you upgrade.

The App Store installer is a 4.77 GB download – yes, it should just fit onto a DVD-R disk, if that is any longer of interest to anyone. Once downloaded, the installer automatically starts itself. Don’t let it continue, as on completion of the installation it will delete itself silently, and you will have no copy of that installer. This is useful for everyone, but particularly for those with multiple Macs to upgrade, as they can use the single copy on each.

When the installer puts up its welcome screen, simply locate it in Applications – it is named Install macOS Sierra – and copy it to a safe place on your startup drive, and on another drive (such as a USB memory stick) for safety. The installer contains the full install disk image named InstallESD.dmg, which is located in the path Contents/SharedSupport/ inside the installer app bundle, if you want to use that to make your own bootable USB stick, perhaps.

Installation starts with a pre-install phase which runs for a minute or two under your existing version of OS X, in which the install payload is probably extracted and decompressed from that disk image. Do not be tricked into thinking that the whole install will only take that minute or so: you should allow at least half an hour, and up to an hour or more, depending on your Mac, etc.

You are then prompted to restart, or to let your Mac restart automatically after a minute. That is the time to quit any other apps such as the App Store, although the installer will happily quit them for you if you prefer.

The main installation will vary according to your Mac and many other factors, and may involve multiple restarts accompanied by startup chimes. On this iMac17,1, it went as follows:

1. startup chime, main install phase with progress bar and time to complete, lasting around 22 minutes, on a black screen
2. no startup chime, but a fresh black screen and new progress bar (no time) lasting about a minute
3. no startup chime, but a fresh black screen with progress bar (no time) lasting another 2-3 minutes
4. no startup chime, but a fresh black screen with a progress bar and time to complete, for a software download which was probably the updated Xcode command line tools (which only Xcode users should experience), followed by a brief completion of the same progress bar
5. new screen inviting you to enter the password for your Apple ID to complete installing iCloud; if you don’t use iCloud you can skip this
6. short setup dialog
7. loading of Finder, etc., without any selection of user or entry of password.

Once Sierra is running, you will find updates to Apple’s Pro apps – Pages, Numbers, Keynote – and iBooks Author, plus voice updates (version 6.0.1) for any voices which you have installed.

Note that, if you are intending to upgrade to Sierra now, there is no point in installing the Safari 10 update, which is only intended for those staying with El Capitan for the moment.

Once Sierra is up and running, Time Machine will start preparing a large backup, given all the files which have been updated.

I wish you success, and a smooth and trouble-free upgrade. I will post further news tomorrow (21 September) and thereafter about Sierra and reports of any significant issues.