Apple Beta Feeds
iOS 12 Beta profile for iPhone/iPad/iPod: link
watchOS 5 Beta profile for Apple Watch:
Your Watch MUST be on the charger during the install.
You CANNOT remove your Watch from the charger during the update. IF YOU DO, YOU WILL BRICK IT
Your phone will tell you when the Watch is done.
Proceed with caution: link
tvOS 12 Beta profile: link
macOS Mojave beta feed installer: link