*** As with all Betas only install on a non production device and computer that you're not concerned about & make backups! ***
These advanced steps are intended primarly for system administrators and others who are familiar with the command line.
You can download the latest macOS Sierra 10.12 Developer Preview here https://developer.apple.com/download
You will need an Apple Developer Account to sign in and download macOS Sierra 10.12 Developer Preview. Public Beta will be out later in July 2016
Use the 'createinstallmedia' command in Terminal to create your bootable usb installer:
- Download macOS Sierra 10.12 Developer Preview from the Mac App Store after redeeming the code from the Apple Developer Download site. Quit the installer if it opens automatically after downloading. The installer will be in your Applications folder.
- Mount your USB flash drive or other volume. You could also use a secondary internal partition.
- Format/Erase your drive and name it "macOSSierra" if you intend on copying and pasting the below command.
- Open Terminal (it is found in the Utilities folder)
- In terminal paste the following command:
sudo /Applications/Install\ 10.12\ Developer\ Preview.app/Contents/Resources/createinstallmedia --volume /Volumes/macOSSierra --applicationpath /Applications/Install\ 10.12\ Developer\ Preview.app --nointeraction
You've now created a bootable macOS Sierra 10.12 Developer Preview USB installer
Down the road when macOS Sierra 10.12 is at full version I would highly suggest using "DiskMaker X". It's simple and easy to use