[Pixel C Q&A, Help & Troubleshooting] m+a+r+k:
Apologies if I'm trying to teach you to suck eggs.

I make no promises, this worked for me. I can't be held responsible for any damage (unlikely) caused when attempting this.

You will need the latest (important) adb and fastboot package. You don't need the rest of the SDK Platform Tools.

https://developer.android.com/studio/releases/platform-tools#download and choose which platform you have. Unpack and copy folder to Program Files (x86) or whatever.

Download latest TWRP from this forum

Download Magisk 17.2 from https://xdaforums.com/apps/magisk

Enable developer mode

Go to Settings > About tablet

Tap on "Build Number" 7 times until it says "You are now a developer"

Enable OEM Unlocking

Go to Settings > Developer options

Enable "OEM Unlocking"

Tap on Enable when a disclaimer pops up

Enable USB Debugging (ADB)

Go to Settings > Developer options

Enable "USB debugging"

Tap on OK when prompted with a disclaimer

If the ADB USB driver has been installed, you will see a popup that says "The computer's RSA key fingerprint is: xx:xx:xx:xx(blah blah)"

Check the box next to "Always allow from this computer"

Tap on OK

Unlock Bootloader

Connect Pixel C to PC with USB power cable.

PC should install drivers (fingers crossed).

Open a Command Prompt window so that it points to the SDK Platform Tools folder.

[Type commands in blue]

Type

adb devices

Do you get a response with a number? If not you don't have a connection and drivers may not be installed or USB Debugging enabled. Windows 7 and 10 recognized Pixel C OK for me.

adb reboot bootloader

The device will now boot into the bootloader and say "Waiting for fastboot command..."

Input the following to unlock the bootloader:

fastboot flashing unlock

You will be prompted on the tablet screen with a warning about unlocking the bootloader.

Press the power button to unlock the bootloader.

The device will reboot and say "Bootloader is unlocked and OS verification is OFF. Device will continue booting in 30 seconds." You will also hear an audible beep. You will then see some spinny stuff. This is the userdata being formatted.

When complete, the device will boot back into the bootloader and say "Waiting for fastboot command..."

Boot back into Android by inputting:

fastboot reboot

Your Pixel C will be reset, so you will need to re-enable USB debugging.

Connect to your wireless network, set up your lock screen password and then skip past everything else in the initial setup, as all of this setup info may be wiped again later on.

Enable "USB Debugging" again.

When you are happy copy the Magisk.zip file to Pixel C Downloads folder.

Install Custom Recovery

Input the following:

adb reboot bootloader

The device will now boot into the bootloader and say "Waiting for fastboot command..."

With the Pixel C in the Fastboot bootloader and the TWRP recovery file in your ADB directory, input the following:

fastboot flash recovery TWRP-3.2.1-0-dragon.img <or whatever the name of the TWRP file>

If OK: THIS IS IMPORTANT If you reboot the device back into Android, it will write over the custom recovery with the stock one. Reboot to TWRP Recovery by holding down "Power button + Volume Down button" until screen turns off, keep pressing Volume Down to boot into Coreboot Bootloader. Navigate to "Reboot into Android Recovery" with Volume Down button and then press Power button to select.

You should now boot into TWRP. You'll be asked for encryption password - it is your lock screen password or if no password has been set use "default_password".

You can now play with settings in TWRP (colour, clock etc).

Make a Nandroid Backup.

Install root

In TWRP select Install menu and select the Magisk zip file. Drag the bar to the right. When finished Wipe Caches.

At main menu Reboot to System and set up your Pixel C.

Install Magisk Manager apk.

Install one of the many free Root Checker apps to check all is OK.

Phew...

I've probably missed something...

Thanks to b1ggjoe and gargamon for being my guinea pigs on this script :)