A Freedom Decrypted tech show video demo of these instructions is available for download.
1a. Step one is to download the assets for the original registered version of doom (note: the source code was released under the GPL license for the software, which we will install later) from Good Old Games (a DRM-free online source for games). While there will be a warning that this is not compatible with Linux that is OK. We'll be installing a different version of the software and merely need to extract the assets from the game. After paying for the game it isn't entirely clear how you download it. To get to the download section go to your user name and click on "Ultimate Doom, The" (under My Collection). Download from the "DOWNLOAD OFFLINE BACKUP GAME INSTALLERS" section to ensure you get everything (ie setup_the_ultimate_doom_1.9_[some_number].exe).
https://www.gog.com/game/the_ultimate_doom
1b. Download the Free SIGIL Megawad (or SIGIL + Buckethead) and extract the SIGIL.wad file to your download directory:
https://www.romerogames.ie/si6il
2. Step two is downloading GZDoom from the below site (it's not in Debian/Ubuntu's own repository so we will have to manually install it). Make sure you select the right version. If you are on a modern system and running Debian/Ubuntu or a derivative distribution like Trisquel or Linux Mint then "Ubuntu (Modern, 64-bit)" is the version of the software you want to download.
3. Step three is opening a terminal and installing GZDoom package and wine (note: you will need to have the universe repository enabled on most distributions if installing on Ubuntu or a derivative distribution).
cd ~/Downloads
sudo dpkg -i gzdoom_4.1.2_amd64.deb
sudo apt-get install wine-stable
4. Step four is to run the MS Windows executable installer using wine (we will not be using the MS Windows version of the software, but will merely use the games assets once the MS Windows version is installed):
wine '/home/user_name/Downloads/setup_the_ultimate_doom_1.9_[some_number].exe'
5. Once everything is installed and/or extracted you can start the game:
gzdoom -iwad '/home/user_name/.wine/drive_c/GOG Games/DOOM/DOOM.WAD' -file '/home/user_name/Downloads/SIGIL.wad'
6. Open the game starts just go to New Game and select SIGIL to play.