Today, Google announced the launch of Android P as a beta at I/O and if you don't want to go to the OTA beta program path, you can get the system images and do the flashing now. The system images are available for download, ready for flashing on your unlocked device. You still need one of the supported Google devices to install the system image for developer preview 2, though.
The OTA and system images both are same, to install the system image you need an unlocked bootloader and ADB set up on your computer to flash the image. By default, this erases your phone. You can turn that off in the batch file, but that can lead to some weird bugs.
Google also announced the second developer preview would run on phones from other companies like OnePlus and Sony. However, the system images are only available for Google's phones. That means you need a Pixel, Pixel XL, Pixel 2, or Pixel 2 XL.
