There are several different ways to do a clean macOS 10.12 Sierra installation. One of simplest and easiest ways is to install via a USB stick.You don’t need to buy a special USB stick nor do you have to purchase a special program to make an macOS 10.12 Sierra USB Install Stick.

Creating a USB Install Stick for macOS 10.12 Sierra stick is very easy to do with these four easy steps.

1. Ensure that you have the Install macOS Sierra.app installer application in your Applications folder. If you don’t have it, you can download it from the App Store. You will need to look in the Purchased tab and click Download for macOS 10.12 Sierra
2. You will need plugin a 8GB or larger USB stick, preferably a USB 3.0 stick. This USB stick will be erased of all data. A faster USB stick will help speed up this step as well as decreasing the time that it will take for macOS 10.12 Sierra to install.
3. Open up Terminal, located in the Utilities folder, which is located in the Applications folder.
4. Run the follow program as listed below:

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

Now you can Quit Terminal and Eject your USB stick. You now have a fully function macOS 10.12 Sierra USB Stick.

If you need more details, please check out the macOS 10.12 Sierra bootable installer page from Apple