CyanogenMod on a Nexus 7 (Wifi 2012)

An alternative OS can speed up the old tablet device.

Don't believe those who claim it is easy. Installing CM for the first time is a challenge. First the obligatory warning: all data and apps will be wiped and I'm not responsible for any damage. Custom ROMs are always an experiment that can destroy your device. I don't know about the backup and restore options. My main motivation was to remove all the pre-installed apps which I suspect to slow down the system and to get control over the permissions I grant to all apps. I did the clean CM 12.1. install in August 2015, maybe things have changed when you read this. But it seems things were easier two years ago. There was a one-click-installer which is being dicontinued. The links to the download sites are dead. At least there is the Nexus Root Toolkit (NRT). Download and install it.

General Preparations:

click 7 times on "Build" in settings/about the tablet. This will grant you developer options
turn un USB debugging in settings>developer mode
connect the Nexus to the PC via USB cord and keep being plugged in
change the USB mode on the Nexus to Camera (PTP)

Rooting

for all actions be patient, it takes some time and rebooting. For these actions you don't need to touch the Nexus. It's all being done remotely.

Run NRT
NRT: check if it shows the correct device (auto detect or manually)
NRT: Full driver installation guide -> Step 4 (full driver test)
NRT: Create Android backup file (Nandroid recovery image won't work until you're rooted)
NRT: Unlock
NRT: Root (check "and also flash custom recovery")

Now the Nexus should boot with an unlocked sign and you can run Superuser (SU) and Busybox. This is a rooted system.

Installing CM

download CM from cyanogenmod.org (grouper) and choose the "build" as a ZIP (not the recovery .img)
decide for which Google apps version you want to go (click on the comparison chart button) and download it. I prefer Pico because everything else can be installed manually afterwards. The Android version must accord to the CM version. (Android 5.1 = CM 12.1)
copy both files into the Nexus root directory (with Windows File Explorer)
turn the Nexus off
turn it on with Power + Volume Down pressed simultaneously. With Vol Up/Down navigate to Recovery and press Power
wipe (default) and confirm
install both files and confirm. Flashing takes a few minutes
Reboot

The first boot takes a while. After 1 minute you see the CM symbol. After 3 more minutes 80+ apps are being adjusted. The system will be ready 10 minutes later. My "nightly" build I chose works without issues. "Snapshots" are better though, "release" and "stable" are really stable.

some notes for phone devices

here it is recommended to update APNs (internet access). The app "APN restore" does not work with Android 4. But it is easy. Just copy /system/etc/apns-conf.xml to your PC or external SD card.