Unfortunately this year Google has retired their official efforts for the NExus 7 (2012) which is starting to show its age with the outdated and aging Tegra 3 Quad-core processor from NVIDIA. Though thankfully this is where the power of Android comes in. Thanks to the openness of the platform that Android is built on Google releases the core code out to the Open Source Community via the Android Open Source Project (AOSP). Upon doing this dedicated devs in this community quickly bring the new features over to their custom ROMs and it always has turned out once Google determines a device unworthy someone from the community believes otherwise and bring the goodness to these types of devices. Now we can bring some new battery enhancements and a refreshed / refined Material UX to our precious Nexus 7 (2012 model) tablets.
Step 0: Install Custom Recovery
Make sure you have a custom recovery installed this will make things a lot easier to manage wiping data, etc.
Latest ClockWorkMod Recovery Images:
[root:clockwork-mod]$ fastboot flash recovery recovery-clockwork-touch-18.104.22.168-grouper.img
sending 'recovery' (7318 KB)...
OKAY [ 0.919s]
OKAY [ 0.278s]
finished. total time: 1.197s
Step 1: Setup Device For Flashing
Enter the recovery menu either via commandline adb reboot recovery or via the bootloader screen fastboot reboot-bootloader then select the "recovery" prompt.
1. Wipe Data / Factory Reset
2. Wipe Cache Partition
3. Advanced / Wipe Dalvik Cache
Step 2: Flash ROM
Download the ROM from the XDA Member and Googler dmitrygr who has been working on bringing new life to this aging tablet by porting the newly available AOSP code pushed by Google shortly after Marshmallow was released. Most of the issues have been documented in his blog and everyday features seem to be working properly even Doze! (source)
adb reboot-bootloader # optional to get into bootloader
fastboot flash boot boot.img # flash the kernal and ramdisk
fastboot fastboot flash system system.img # flash system image
Step 3: Setup SuperSU and Gapps
Since this device shipped with Google Play it is legal to install the gapps package on this device. Thanks to the XDA Community member zephik for providing the Benzo Gapps package I used for this install (source).
Reboot your device (already done from last step) into recovery. Select "Install ZIP > From Sideload" to sideload the gapps package from your computer using ADB via the following command.
adb sideload benzo-gapps-M-20151011-signed-chroma-r3.zip
From recovery go ahead and wipe data, cache and dalvik cache and restart to power into Android. This may take up to 20 minutes (depending) to optimize and initialize your device for the first time.