[Nexus 6  General] Harry44:
After many hours of testing, here it is.

[COLOR="Red"]Your warranty is now void.
I am not responsible for anything that may happen to your device as a result of this![/COLOR]

Install custom roms and use Project Fi network with network switching.

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[IMG="T-Mobile"]https://tctechcrunch2011.files.wordpress.com/2011/08/t_mobile_logo.jpg[/IMG][IMG="Sprint"]https://upload.wikimedia.org/wikipedia/en/thumb/4/48/Sprint_Nextel_logo.svg/294px-Sprint_Nextel_logo.svg.png[/IMG]

What is required:
*Nexus 6
*Project [COLOR="Lime"]F[/COLOR][COLOR="Yellow"]i[/COLOR] sim card
*Access to a computer
*ADB and fastboot installed
*Time and patience

Lets get to it...(Skip steps if they already apply to you.)

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1. Unlock your bootloader

1. Enable “Developer options” by tapping 5 or 6 times on the Build number in Settings>About phone.

2. Hit back once and choose Developer options; check the box for USB debugging.

3. While in there, also check the box for “Enable OEM unlock.”

4. Plug your phone into your computer.

5. You’ll need to give it USB debugging access through a pop-up (don’t forget to check the box).

6. Once you have given your computer debugging access, it’s time for commands.

7. Open a command prompt from within the Platform-tools SDK folder where your adb and fastboot files are.

8. Type the following command: "adb reboot bootloader"

10. A bootloader unlock confirmation page will appear. Tap Volume Up to highlight “Yes,” and Power to select it.

11. Your phone’s bootloader will now be unlocked. Shouldn’t take more than a minute or two.

12. Once finished, you should be on the boot screen showing “Start.” Press Power to reboot your phone.

13. During reboot, your phone will go through a factory reset.

14. Once it boots back up, you are done. If everything went correctly, you should see an unlock icon during boot.

THANK YOU DROIDLIFE FOR THESE DIRECTIONS http://www.droid-life.com/2014/11/21/how-to-unlock-the-nexus-6-bootloader/

15. Activate your sim card.

2. Download and Install Project Fi's specific factory image

1. Navigate your web browser to https://developers.google.com/android/nexus/images?hl=en

2. Click on "Link" where it says 5.1.1 "(For Project Fi ONLY) (LVY48E)", this will download the factory image.

3. After it has downloaded, navigate to your downloads folder. Right click on shamu-lvy48e-factory-5e7c69da.tgz and extract.

4. Navigate to the newly created "shamu-lvy48e-factory-5e7c69da" folder.

5. Right click and extract "shamu-lvy48e-factory-5e7c69da.tar".

6. Navigate to the newly created "shamu-lvy48e-factory-5e7c69da" folder.

7. Click on "shamu-lvy48e".

8. Right click and extract "image-shamu-lvy48e"

Alright, everything is extracted and you are ready to flash!

10. Run the following commands.

fastboot flash bootloader C:/image-shamu-XXXXXX/bootloader.img
fastboot reboot-bootloader
fastboot flash radio C:/image-shamu-XXXXXX/radio.img
fastboot reboot-bootloader
fastboot flash system C:/image-shamu-XXXXXX/system.img
fastboot flash userdata C:/image-shamu-XXXXXX/userdata.img
(Note: this command will wipe your device (including /sdcard), EVEN if your bootloader is already unlocked.)

fastboot flash boot C:/image-shamu-XXXXXX/boot.img
fastboot flash recovery C:/image-shamu-XXXXXX/recovery.img
fastboot erase cache
fastboot flash cache C:/image-shamu-XXXXXX/cache.img

- After everything finished, select "Recovery" using the volume buttons.

- When a small Android appears, press Power Button + Volume UP.

- Select "Wipe data/factory data reset"

- Reboot. You're now on the Project Fi Factory Image.
Thank you @Mr hOaX and other contributors for your guide on flashing factory images [URL="http://xdaforums.com/nexus-6/general/guide-flash-factory-images-nexus-6shamu-t2954008"]http://xdaforums.com/nexus-6/general/guide-flash-factory-images-nexus-6shamu-t2954008[/URL]

3. Download and install TWRP and root

1. Navigate to https://dl.twrp.me/shamu/

2. Click the latest TWRP file. In my case it is "twrp-2.8.7.1-shamu.img"

3. Navigate to https://download.chainfire.eu/696/SuperSU/UPDATE-SuperSU-v2.46.zip

4. Download the latest SuperSu.zip

5. Place the SuperSu.zip on the root of your device storage.

6. Reboot to bootloader.

7. Run the command "fastboot flash recovery twrp-2.8.7.1-shamu.img

8. You now have a custom recovery.

9. Run the command "fastboot reboot recovery

10. Flash the latest SuperSu.zip that you placed on the root of internal storage.

11. Congratulations you're now rooted with a custom recovery!

4. Download and install MultiRom

1. Navigate to http://xdaforums.com/nexus-6/orig-development/multirom-t3074529

2. Follow the instructions provided by [MENTION=3418703]Tasssadar[/MENTION] (Make sure to install the 5.1.1 kernal patch)

3. Come back here after everything is completed.

5. Install your favorite rom

1. Follow the directions on how to create a rom slot and install your new rom. http://xdaforums.com/nexus-6/orig-development/multirom-t3074529

YOU'RE ALMOST DONE.

6. Here is how you can switch back and forth between the T-Mobile and Sprint

1. If you are not booted into the internal stock rom, boot into it now.

2. Open dialer.

3. Choose which carrier you want to make your primary carrier.

Sprint = *#*#34777#*#* or *#*#FI SPR#*#*

T-Mobile = *#*#34866#*#* or *#*#FI TMO#*#*

4. Open up dialer and type in one of the codes you see above, your choice of code depending on carrier.

5. After your phone has restarted the network connection you are safe to reboot into your favorite custom rom (I tested this with CM 12.1)!

Reboot and switch any time you like!

Want to go back to using the stock rom with Project Fi's auto network switching. Use the following dialer codes:

Fi Auto Network Switching = *#*#342886#*#*

then

Fi Repair = *#*#34963#*#*

Thank you to [MENTION=519243]maejrep[/MENTION] for originally posting the dialer codes!

I'm working on making an app to automate network selection but you have to stay tuned for that in a post update!

A BIG THANKS TO EVERYONE THAT HAVE MADE THE CONTRIBUTIONS TO XDA THAT MADE THIS POSSIBLE. CREDIT IS POSTED WHERE ITS DUE.

Donate to me!​