I just installed successfully! Here's the procedure I went through:

• Download and extract the images. You should get the Bootloader (boot.img), Android 6 (system.img) and the Radio (n4_lte_modem.img, normal_modem)

• Also I recommend downloading a custom recovery image like TWRP recovery (handy for backups, installing Gapps and wiping if needed).

• On Windows, if you don't have it already install Universal ADB driver and Mini ADB and Fastboot

• Make sure you backup all important data in case something goes wrong. I recommend doing a factory reset after flashing in any case.

• Start phone in fastboot mode (power off, then hold Power + Volume down) and connect the USB cable

• Starting Mini ADB and Fastboot gives you a command line at "C:\Program Files (x86)\Minimal ADB and Fastboot".

• To flash Android 6, get the paths to your extracted img files and pick one of your radio images, depending on if you want LTE enabled or not. Then type these commands, modified to use your image paths obviously.

  fastboot flash boot "C:\Path\to\boot.img" fastboot flash system "C:\Path\to\system.img" fastboot flash radio "C:\Path\to\n4_lte_modem.img"

• To install and launch TWRP, execute the following command. Then reboot the device into Fastboot again and select Recovery mode:

  fastboot flash recovery "C:\Path\to\twrp-2.8.7.0-mako.img"

• To install Gapps, while in TWRP go to Advanced / ADB Sideload and swipe to start Sideload. You may have to manually install the "Android ADB Interface" drivers through Windows Device Manager now. then type this command:

  adb sideload "C:\Path\to\open_gapps-arm-5.1-stock-20151009.zip"

• If you want a fresh install, you can then use TWRP to perform a factory reset. This will delete all your data and apps - make sure it's backed up. Tap Wipe and swipe to factory reset.
  Android 6 should boot without the factory default, keeping your apps and data, but I didn't test that.