One of the many benefits of getting a Nexus device, such as the Nexus 6, is the ability to easily root it and further customize the device's operating system. Rooting, aka the act of gaining administrative rights, allows you to make changes to the device's software that you normally wouldn't be allowed to make.
One such change comes from the ability to flash custom ROMs. Custom ROMs, or custom versions of the operating system, allow you to flash an entirely new, more customizable version of the OS onto your Nexus 6. After rooting the Nexus 6, you can then flash a custom recovery, and finally a custom ROM. You can even flash multiple custom ROMs until you find one that suits your needs.
If any of that sounds as exciting to you as it does to me, fire up your Windows PC and get started with these steps. Note that this will erase your device so be sure to backup anything you need to keep to your computer.
1. Download said toolkit from the developer's page here and save it to your desktop.
2. Right-click the toolkit and click Run As Administrator.
3. Click Install and follow the prompts to complete the installation.
4. On your device, go to Settings > About Phone.
5. Scroll down to Build and continuously tap on it until it says you are a developer.
6. Go to Settings > Developer Options and turn on Enable OEM Unlock & USB Debugging.
7. Plug the device in via USB.
8. Tap Always Allow from this Computer and tap OK.
9. Tap Unlock in the toolkit to unlock the bootloader.
10. Using the volume buttons to navigate and power to select, choose Yes on the Nexus.
11. Go through the normal setup process on the device to get it back to the home screen.
12. On your device, go to Settings > About Phone.
13. Scroll down to Build and continuously tap on it until it says you are a developer.
14. Go to Settings > Developer Options and turn on Enable OEM Unlock & USB Debugging.
15. Under Root in the toolkit, check Custom Recovery and click Root.
16. Open SuperSU on the phone and allow it to update if necessary. You're rooted!
David Cogen's site, TheUnlockr.com, is a popular site for rooting, jailbreaking, hacking, modding, and other how to's to show you how to unlock your device's true potential. Head to his ROM repository for ROMs you can now flash onto your Nexus 6 since you're rooted.