[Nexus 6P General] theorie:
Disclaimer: I am not a developer. I am posting this to help others who are trying to achieve the same results. I wasted a few hours of time trying various sequences of installation, different files, etc., until i finally got it right. Posting this up so it hopefully helps someone else!

Various problems I was running into:

Camera would stop working (any app) and show only the bottom buttons, a black screen and a camera icon in the middle.

Snapchat wouldn't let me login.

Snapprefs wouldn't work.

Android Pay wouldn't work. (Heads up: you cannot have Xposed and Android Pay both working...it's not possible as of 2016/01/29. If ever and when it is...and I test it myself, I'll update this thread.)

Here are the steps I took to get the phone Systemless rooted, get Xposed functioning, and then get SnapPrefs working (but no Android Pay).

Note: I am not providing step-by-step detailed instructions -- these are more of the general process. Make sure to read all of the information from the sources I reference.

Disclaimer: This worked for me, but I cannot guarantee this will work for everyone. I am not responsible for anything that happens to your phone if you try to repeat these steps. Also, this probably voids your warranty.

Hat tip to the following threads for information on the various stages of the process, developers for their hard work, and everyone else who has put up any info about this subject!

[GUIDE] Unlock/Root/Flash for Nexus 6P

[Help] Unfortunately, Nfc service has stopped

Files Used:

Nexus 6P Factory Image 6.0.1 (MMB29P) --- "angler-mmb29p-factory-25ed9560.tgz"

TWRP Recovery for angler 2.8.7.0 --- "twrp-2.8.7.0-angler.img"

Systemless SuperSU 2.67 --- "BETA-SuperSU-v2.67-20160121175247.zip"

Xposed 3.0 Alpha --- "XposedInstaller_3.0_alpha4.apk"

Xposed Framework v79 SDK23 for ARM64 --- "xposed-v79-sdk23-arm64.zip" (optional: grab "xposed-uninstaller-20150831-arm64.zip" while you're there)

Snapchat 9.21.1.0 --- "com.snapchat.android_9.21.1.0-771_minAPI16(armeabi-v7a)(nodpi).apk"

Snapprefs 1.6.5 (No need to download -- you can install via Xposed app)

Steps I took (NOTE: Do

not

allow TWRP to install root when it asks, "Install SuperSU now?" when exiting Recovery. Always click "Do not install."):

Optional: Flash the latest official system image from Google (MMB29P). I did because I had trouble with previous attempts and wanted to start fresh.

Unlock bootloader using Step 1 from this guide.

Download TWRP 2.8.7.0 from above link, then install using Step 2 from this guide.

Optional: Follow Steps 3 - 6 from this guide. You will now have Systemless Root.

Reboot into Android, use Play Market to install a file browser (I prefer Astro) then browse-to and install the Snapchat 9.21.1.0 apk. Once it's done, open and log-in to Snapchat. Make sure you are fully logged-in and can view/send snaps.

Use the file browser again to browse-to and install Xposed 3.0 Alpha apk. Once it's done, open it, go to Settings and change Installation mode to "Recovery (flash zip automatically)".

Open SuperSU, go to Settings, and check the box next to "Enable su during boot" (if you don't do this, when you reboot, NFC service will enter a neverending crash loop).

Reboot into Recovery, install Xposed Framework v79 SDK23 for ARM64, reboot. (Remember: click "Do not install," when prompted to install SuperSU.)

After this it will take a few minutes to do the "Optimizing app # of #" thing.

Once rebooted, open Xposed search-for and download Snapprefs 1.6.5. After installed, enable the Snapprefs module in Xposed, then reboot.

Once rebooted, open Snapprefs and customize your settings. Set your save location, etc.

That's it. You should now have functioning root, functioning Xposed, logged-in Snapchat, and functioning Snapprefs.

I know some people are going to say, "It's pointless to do Systemless root since Xposed modifies /system and breaks Android Pay," but

the reason I went this route is because I normal rooting kept breaking my Camera (in any app)

. Maybe in the future there might be some way to keep Android Pay functioning and run Xposed modules without modifying /system...who knows.