New Installation Instructions

Install

Prerequisites

You should have at least 2GB of free memory available on the PC.
You need the unlocked variant of one of the supported devices, not a locked carrier specific variant.
It's best practice to update the stock OS on the device to make sure it's running the latest version before proceeding with these instructions. You can just connect the device to the internet, and let it auto update to the latest version available. You can check for updates by opening the Settings app, then near the bottom tapping System ➔ Advanced ➔ System Update.

Obtaining fastboot

You need an updated copy of the fastboot tool and it needs to be included in your PATH environment variable.

You can download the official releases of platform-tools from Google. You can either obtain these as part of the standalone SDK or Android Studio. For one time usage, it's easiest to obtain the latest standalone platform-tools release, extract it and add it to your PATH in the current shell. For example:

unzip platform-tools_r29.0.2-linux.zip
export PATH="$PWD/platform-tools:$PATH"

Sample output from fastboot --version afterwards:

fastboot version 29.0.2-5738569
Installed as /home/username/downloads/platform-tools/fastboot

Don't proceed with the installation process until this is set up properly in your current shell. Also, make sure the version matches the above or is newer/higher than this, as older versions are unsuitable for flashing.

Enabling OEM unlocking

OEM unlocking needs to be enabled from within the operating system.
Enable the developer settings menu by going to Settings ➔ System ➔ About phone and tapping on the build number menu entry until developer mode is enabled.Next, go to Settings ➔ System ➔ Advanced ➔ Developer settings and toggle on the 'Enable OEM unlocking' setting. This may require internet access on some devices

Unlocking the bootloader

First, reboot into the bootloader interface. You can do this by turning off the device and then turning it on by holding both the Volume Down and Power buttons.
The bootloader now needs to be unlocked to allow flashing new images:

fastboot flashing unlock

The command needs to be confirmed on the device, using the volume and power buttons.

Flashing factory images

Reboot into the bootloader interface to begin the flashing procedure.
Next, extract the factory images and run the script to flash them. Note that the fastboot command run by the flashing script requires a fair bit of free space in a temporary directory, which defaults to /tmp:

unzip crosshatch-factory-2019.06.23.05.zip
cd crosshatch-pq3a.190605.003
./flash-all.sh

Use a different temporary directory if your /tmp doesn't have enough space available:

mkdir tmp
TMPDIR="$PWD/tmp" ./flash-all.sh

Wait for the flashing process to complete and for the device to boot up using the new operating system.
You should now proceed to locking the bootloader before using the device as locking wipes the data again.

Re-locking the bootloader

Re-locking the bootloader is important as it enables full verified boot. It also prevents using fastboot to flash, format or erase partitions. Verified boot will detect modifications to any of the OS partitions and it will prevent reading any modified / corrupted data.
In the bootloader interface, set it to locked:

fastboot flashing lock

The command needs to be confirmed on the device since it needs to perform a factory reset.
Unlocking the bootloader again will perform a factory reset.

Disabling OEM unlocking

OEM unlocking can be disabled again in the developer settings menu within the operating system after booting it up again.

At this point, your device should be all setup and ready for usage with CalyxOS.

Replacing CalyxOS with the stock OS

Installation of the stock OS via the stock factory images is the same process described above. However, before locking, there's an additional step to fully revert the device to a clean factory state on modern devices like the Pixel 2, Pixel 2 XL, Pixel 3, Pixel 3 XL, Pixel 3a and Pixel 3a XL.
This step needs to be performed by rebooting to the bootloader like before, and then running:

fastboot erase avb_custom_key