Installing Nougat has never been easier. Procedure described in post 239 of this thread.
Development for unofficial CM-12.1 for Nook HD and Nook HD+ has ceased.
The author @amaces has moved on to Marshmallow (Android 6), and the zip files for these progressive releases are what you now see at the collaboration link. If you wish to install CM-12.1 look instead through the pages of his "obsolete" folder for "cm-12.1-20151018" and "twrp-220.127.116.11" final releases. CWM should install these properly but later versions are likely to fail due to deficiencies in the CWM recovery utility.
Better yet try the latest Marshmallow and TWRP versions. For this you must create a new bootable microSD card using these files provided by @belfastraven and the downloaded zip files "cm_hummingbird-ota-MHC19Q.160407.zip" and "twrp-3.0.1-0-hummingbird.zip". These versions may advance by the time you happen to do this. The procedure is the same as described in the .pdf guide for CM-12.1, except with the new files.
And use a current GApps file for the ARM platform, Android 6.0 from http://opengapps.org/.
This is a detailed tutorial for beginners. Seasoned users may find it overly verbose.
My toy box contains some Nook HD and Nook HD+ tablets, and I recently became aware of CyanogenMod. I studied about it for a while and finally tried a CM-12.1 installation. It was successful, and I was so impressed by the improvements that I told some Nook-owning friends about it. They quickly decided to do likewise and asked for instructions.
My friends and I are all retirees, so we have seven Saturdays a week to spend as we wish. I decided to spend a few of mine re-writing my notes into an instruction manual. As of today, September 25, 2015, there are eleven formerly stock Nook tablets whose beginner-owners have followed the instruction and successfully installed CM-12.1. Several of these are being regularly updated as revisions are released. No bricks have been cast so far.
During the study period I spent a lot of time on xda developers pages, and it eventually occurred to me that there might be other beginners who could make good use of Nook-specific instructions. So I am pleased to offer this manual to anyone interested, and hope it will save you some time and trouble.
The procedure uses the technique and boot files by @leapinlar. The ROM and TWRP zip files used are those created by @amaces. Profound thanks to these experts for their diligent work and generosity.
Below is a synopsis of the instructions. The complete PDF document is attached to this post.
This document will guide you through the steps of installing a pure modern version of the Android operating system on your Nook HD or Nook HD+ tablet. The installation is done from a bootable microSD card using the ClockWorkMod recovery utility to install the contents of zip files. This straightforward method does not require ADB or rooting the Nook. The result is CM-12.1 installed with basic Google apps and your choice of TWRP or CWM for your resident recovery utility.
There's room for improvement.
If I could learn how to create a bootable microSD that would boot to TWRP instead of CWM the procedure could be reduced to four easy steps. I have found no help for this, and my own attempts have all failed. I would be most grateful for any help so I can update the instructions.