Here are the full steps required for flashing - I write them here because some of these are at risk to disappear from the internet for good after the demise of CyanogenMod wiki.
Note: THIS WILL WIPE YOUR DEVICE, YOU WILL LOSE EVERYTHING THAT IS ON IT so first make backups etc etc.
First you need to install the Android tools. I recommend doing this with homebrew, install it if you don't have it yet (it's extremely useful in many many cases).
Then open a terminal on your mac and type:
- brew tap caskroom/cask
- brew tap caskroom/versions
- brew cask install java8
- brew cask install android-sdk
Back to Settings, tap on Developer Options and enable USB Debug mode and Stay Awake.
Go back to your mac terminal and type:
- sudo adb start-server
Connect the tablet with a real USB cable (beware charging-only cables! Those won't work.)
On the tablet screen, you should be prompted to authorize the device; do it.
Back to terminal:
- adb reboot bootloader
- fastboot oem unlock
If it doesn't reboot on its own, select Start (again with the power button). At this point you should see an unlocked pad at the bottom of the screen as Android loads.
Note: if you plan to flash another OS, there is no point in going through the whole setup at this point, skip as much as you can.
Re-enable Developer mode and USB Debugging.
At this point the machine is rooted. The following steps are necessary only if you want to install LineageOS or other hacks; at the very minimum, though, you should install Trimmer (fstrim) from the Play store and use it liberally. Anyway, on with the flashing...
Get the latest image for "grouper" from https://twrp.me/, and unzip it.
Back to the terminal:
- cd folder/where/you/have/your-downloaded-image.img
- fastboot flash recovery your-downloaded-image.img
- adb reboot bootloader
Get the image you want to install. I'm currently trying this but in general anything after CM10 / Android 4.1 may be on the slow side for the original Nexus. You probably want the Google Apps from opengapps.org - choose ARM / 7.1 and the Micro option. Save both the apps zip and the image zip in the same folder.
Back to Terminal, let's copy these files to the device:
- cd folder/where/you/have/your/zips
- adb push your-downloaded-image.zip /
- adb push your-downloaded-gapps.zip /
- adb push gapps-config.txt /
Install the Trimmer (fstrim) app, which is absolutely necessary. Launch it and click Trim Now. Click on the settings icon and enable autotrim as frequently as possible. In fact, every time you install an app or write a bunch of files, before you launch the new app, open Trimmer and do a trim, or it will all get laggy again.
Another good app is Android SSH Server, although it's a bit old it still works fine and it's easy to configure. You just have to use the options -t -oHostKeyAlgorithms=+ssh-dss -p XXXXX (where XXXXX is the custom port configured in the app, Android won't allow the classic one) when connecting.
And that's it.