Installing macOS 10.12 via USB
When faced with a new version of your operating system you typically have two choices. Upgrade your existing installation or format the drive and do a clean install. Most people are going to opt to do an upgrade and that's perfectly fine. In fact, that's how I recommend doing it. However, for the hardcore amongst us, there's simply no substitute for a clean install.
Creating a bootable macOS 10.12 Sierra USB drive is actually easier than you might think. All you need is the Sierra installation file (Install macOS Sierra.app), which you can get from the mac app store, and any 16GB USB thumb drive. The installation app will format your drive and then copy over the operating system files files, so make sure the drive you're using doesn't have anything on it that you want.
Ready to get started?
Start off by inserting the USB drive and wait for its icon to appear on the desktop. Now right-click on the icon, choose "Rename", and change the name to Untitled. Next, you'll want to download the Sierra installation file from the mac app store. Once the installer has finished downloading it will launch and open an installation window. Click on "Install macOS Sierra" in the menu bar and then "Quit Install macOS." Finally, launch the Terminal app, paste the following, and then hit ENETR.
$ sudo /Applications/Install\ macOS\ Sierra.app/Contents/Resources/createinstallmedia --volume /Volumes/Untitled --applicationpath /Applications/Install\ macOS\ Sierra.app --nointeraction
This part is going to take some time to finish. How long it takes depends on the speed of the drive. A decent USB 3.0 drive will finish in a few minutes. Slower drives can take as much as 10-15 minutes. Once finished, you'll want to reboot your Mac while holding down the "C" key. This will bring up the OS boot menu which will allow you to choose which drive you want to boot from. Choose your new macOS Sierra USB drive and click OK. If everything goes well you'll soon see an installation window. From there you can use the Disk Utility to format your Mac's HD/SSD and then do a clean install.
That's all there is to it! Please let me know if you found this article useful and/or interesting. Thanks!