Download the right image for your device type (ARM64 for most of you) and partition type.
You can either flash a pure AOSP ROM such as phh-Treble, or if you prefer more features you can grab other ROMs like LineageOS 15.1 or Resurrection Remix GSIs. Download the GSI of your choice onto your PC.Just do a quick Google search for “XDA unlock bootloader” + your device name and you should find many guides. We have many guides available both on the Portal and in our forums for you to peruse. The steps you take here are different depending on your device. Guide to flash GSI on devices that support Project Treble Preparation for devices that officially support Treble You can re-install these afterwards, but please be sure you are using the stock boot/ramdisk before proceeding. Your device is free of any heavy modifications such as the Xposed Framework, SuperSU, or Magisk.
Again, refer to this article for a list of such devices.
After-market custom ROMs (customized versions of Android software that are made by independent developers usually not affiliated with a company) are a big draw to the XDA forums, and thanks to the changes in Android that Project Treble require, Treble-supported devices will have an easier time flashing custom ROMs based on the Android Open Source Project (AOSP).