Install Cyanogenmod:

https://wiki.cyanogenmod.org/w/Install_CM_for_grouper

Downloads:

http://download.cyanogenmod.org/?device=grouper

Instructions for OEM unlock:

BE AWARE, THIS WILL WIPE ALL DATA!

1) On Linux, make sure fastboot and adb is installed.

sudo apt-get install android-tools-fastboot android-tools-adb

2) Enable USB Debugging on device

3) Connect nexus to pc via usb and confirm the popup message allowing access

4) adb devices shows the id of the connected device and tests if adb is working

5) reboot device into bootloader (fastboot mode) using

adb reboot bootloader

6.) check if the computer can access the device in fastboot mode using

fastboot devices

7.) unlock the bootloader using (if necessary)

fastboot oem unlock

this will wipe the device and reboot. USB debugging will need to be re-enabled if it does.

Instructions to install custom recovery:

1) Follow steps 1-6 of OEM unlock

2) download TWPR recovery (or official cyanogenmod recovery for the device)
https://dl.twrp.me/grouper/twrp-2.8.7.0-grouper.img

3) Install the recovery image using:

fastboot flash recovery yourrecoveryimagename.img

Instructions to install custom rom:

1) follow steps 1-4 of OEM unlock (not in fastboot)

2) If the custom rom is already on the device (sdcard), if in fastboot, use vol up/down to go to recovery, then press power button to confirm.

If the custom rom is NOT on the device, reboot into normal system, then push custom rom as well as gapps to /sdcard/ folder on device using adb

adb push filename.zip /sdcard/

http://opengapps.org/?api=5.1&variant=nano

3) once done, reboot into bootloader using press volume up, volume down and power for 10 seconds

4) flash cm recovery again using

fastboot flash recovery yourrecovery.img

5.) select recovery mode using volume up/down, then power to select

6.) select wipe cash, then wipe/data factory reset

7.) apply update, from internal, from 0, select main cm image

8.) apply update, from internal, from 0, select gapps package

9.) reboot system now

enable developer mode,

enable root

install supersu (chainfire)

update su via cwm

http://wccftech.com/update-and-root-nexus-9-on-android-6-mra58k-marshmallow/

enable adb debugging and oem unlock (if available), connext nexus 10 via usb, confirm message on nexus10

adb reboot bootloader

if the bootloader is locked, reboot and do a full backup beforehand as unlocking will wipe it.

adb backup -all -f nexus9.ab

then repeat while unlocking the device using

adb reboot bootloader
fastboot oem unlock

confirm with volume up, this will erase the whole device!
reboot
re-enable developer mode and usb debugging

adb restore nexus9.ab

confirm recovery on tablet

adb reboot

download elemental / supersu / twrp

adb reboot bootloader

fastboot flash recovery twrp-2.8.7.0-flounder.img 

select hboot, then boot recovery

enable adb sideload

adb sideload ElementalX-N9-3.00.zip

enable adb sideload

adb sideload BETA-SuperSU-v2.49.zip

reboot system