I searched yesterday for a concise set of instructions for installing the drivers necessary to make the Arduino IDE compatible with non-packaged microcontroller boards and was unable to find a complete solution.
Thus, once I figured out what to do, I thought that I would post the solution to the forum for anyone else that suffers from a similar struggle. This instruction will not work for custom packages, though.
Some computers are either not connected to the internet, or are behind a firewall (company policy, etc.). This prevents the IDE from easily downloading the files necessary to update the package_index.json file as well as the individual files needed for installing the desired packages.
In some capacity, one must have internet access in order to manually install packages.
In order to perform this process manually, one must first manually update the package_index.json file.
Download the following two files:
https://downloads.arduino.cc/packages/package_index.json
https://downloads.arduino.cc/packages/package_index.json.sig
and place them in the following folder on your computer (replace the old files here):
C:\Users{user}\AppData\Local\Arduino15\
Then, re-open the Arduino IDE and navigate to
Tools -> Board:[...] -> Boards Manager...
And search to find the package that you want to install.
Command the IDE to install the package, and it will report an error message in the main window. Within that error message will be a line stating something like:
"Error downloading http://downloads.arduino.cc/cores/megaavr-1.8.5.tar.bz2"
copy that URL to your browser and download the file manually.
Then, place this file into
C:\Users{user}\AppData\Local\Arduino15\staging\packages
If those folders do not exist, create them. I am actually not sure that those folders are necessary, but I did not try a different way and this way worked.
Repeat this process, downloading all of the necessary files to install the package of interest.
Then, once all files are present, the IDE will succeed in installing the package.