You will need the FreeDesktop.org 21.08 SDK installed, if you don't have it, install Flathub which provides it.
First, go to your Steam library, right click on Sonic CD and click Manage > Browse local files.
Copy the Data.rsdk file and the videos folder into this directory.
To build and install the flatpak, run:
System-wide:
$ sudo flatpak-builder --install --force-clean soniccd com.sega.SonicCDSteam.json
User:
$ flatpak-builder --user --install --force-clean soniccd com.sega.SonicCDSteam.json
This method will not install the video files, as the decompilation only supports video files from the Steam version. This method will also not include music or sound effects, as the RSDK file from Origins does not contain them.
First, you need to install Sonic Origins. To get it, visit https://store.steampowered.com/app/1794960 for the Steam version or https://store.epicgames.com/en-US/p/sonic-origins for the Epic Games version.
Once you have the game, you need to navigate to the game's files.
If you have the Steam version, go to your Steam library, right click on Sonic Origins and click Manage > Browse local files.
Once you're in the game's files, navigate to image/x64/raw/retro. Copy the SonicCDu.rsdk file into this directory.
To build and install the flatpak, run:
System-wide:
$ sudo flatpak-builder --install --force-clean soniccd com.sega.SonicCDOrigins.json
User:
$ flatpak-builder --user --install --force-clean soniccd com.sega.SonicCDOrigins.json
This method will not install the video files, as the decompilation only supports video files from the Steam version.
First, you need to install the game on an Android device. To get it, visit https://www.sega.com/games/sonic-cd.
Once you have the game, use a file manager on your device to navigate
to the directory Android/obb/com.sega.soniccd.classic and copy the
patch.[number].com.sega.soniccd.classic.obb file into this directory as Data.rsdk.
To build and install the flatpak, run:
System-wide:
$ sudo flatpak-builder --install --force-clean soniccd com.sega.SonicCDAndroid.json
User:
$ flatpak-builder --user --install --force-clean soniccd com.sega.SonicCDAndroid.json