If you would like to perform a clean install of macOS or update multiple machines, it's a good idea to create a bootable USB installer. The instructions vary just slightly based on which version you're trying to setup.
Download macOS Sierra from the App Store
The installer program should automatically open after the download completes. Do not proceed with the installation, cancel and close it.
Prepare the USB drive
Plugin the USB drive, launch the Disk Utility app, select your USB drive on the left and click the erase button. You can leave the default name Untitled since it will be renamed automatically in the next step. If erasing the disk fails on the first attempt, try it again and it should work on the second attempt.
Setup the install media
Run the program to create the install media from the Terminal.
# For macOS 10.13 High Sierra, 10.14 Mohave, and 10.15 Catalina sudo /Applications/Install\ macOS\ Catalina.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume # for macOS 10.12 Sierra and older sudo /Applications/Install\ macOS\ Sierra.app/Contents/Resources/createinstallmedia --volume /Volumes/Untitled --applicationpath /Applications/Install\ macOS\ Sierra.app
Reboot into the installer
Hold down the option key immediately after you restart the computer. Then select the USB drive with the installer on it and the macOS installer will start.