So I figured out how to do this manually on Windows, and I'm sharing the instructions to help anyone else who might need it out. Might be possible to automate with a batch file but I'm not sure how to set those up.
SETTING UP:
1. Download SirFluffDev's GitHub repository by visiting the link earlier in this thread, clicking on Code, then Download ZIP. This will download ps1-3ds-forwarder-master.zip.
2. Unzip it. I personally renamed the folder from ps1-3ds-forwarder-master to ps1-3ds-forwarder and created a folder in it named assets to hold the banner image, banner sound, and icon.
3. Delete everything in the resources folder except for retroarch.cfg, retroarch_3ds.elf, and template.rsf.
4. Download the Windows versions of 3dstool, bannertool, ctrtool, and makerom and put them in the resources folder. These will all be executables, but 3dstool will also have two text files named ext_key.txt and ignore_3dstool.txt.
BUILDING YOUR GAME:
Some of the paths may be different for you, depending on where the tool is located on your PC and what your files are named. Before you begin, you need to determine these aspects of your game: Long name, short name, publisher, and Title ID.
Long name is the full name of the game (i.e. Revelations: Persona).
Short name is a shortened name of the game (i.e. Persona 1). If your game doesn't have a shortened name, reuse the long name.
Publisher is the company that published your game (i.e. Atlus).
Title ID is a four-digit hexadecimal number (i.e. FB1D). This must be unique, as installing a game with a conflicting Title ID will overwrite any existing installed games with that same Title ID.
1. Copy the retroarch.cfg file from the resources folder to the romfs folder and open this copy in a text editor such as Notepad or Notepad++.
2. Use the editing tools (i.e. Edit > Find & Replace) to replace all instances of ROMNAME with the long name of your game.
3. Make any other edits you want or need to make to the configuration file at this time, then save and close it.
4. If you want to see the PlayStation boot logo when starting the game, open core-options.opt in the text editor and change pcsx_rearmed_show_bios_bootlogo to "enabled", then save and close it.
5. Copy your PlayStation 1 disc image (which must be in the CHD format) to the romfs folder and rename it to game.chd.
6. Press the Windows key and R key to open the Run dialog, then enter cmd to open the Command Prompt.
7. Navigate to the ps1-3ds-forwarder's resources folder. If it's on a different drive, enter the drive letter and a colon (i.e. D:) to navigate to it, then use the cd command to navigate to the folders (i.e. cd ps1-3ds-forwarder then cd resources).
8. Enter the following commands (I'm using Revelations: Persona as an example with a Title ID of FB1D), remember to adjust the paths to match the files on your PC:
9. Afterwards, you can clean up by deleting icon.icn, banner.bnr, and romfs.bin from the resources folder, and retroarch.cfg and game.chd from the romfs folder.
SETTING UP:
1. Download SirFluffDev's GitHub repository by visiting the link earlier in this thread, clicking on Code, then Download ZIP. This will download ps1-3ds-forwarder-master.zip.
2. Unzip it. I personally renamed the folder from ps1-3ds-forwarder-master to ps1-3ds-forwarder and created a folder in it named assets to hold the banner image, banner sound, and icon.
3. Delete everything in the resources folder except for retroarch.cfg, retroarch_3ds.elf, and template.rsf.
4. Download the Windows versions of 3dstool, bannertool, ctrtool, and makerom and put them in the resources folder. These will all be executables, but 3dstool will also have two text files named ext_key.txt and ignore_3dstool.txt.
BUILDING YOUR GAME:
Some of the paths may be different for you, depending on where the tool is located on your PC and what your files are named. Before you begin, you need to determine these aspects of your game: Long name, short name, publisher, and Title ID.
Long name is the full name of the game (i.e. Revelations: Persona).
Short name is a shortened name of the game (i.e. Persona 1). If your game doesn't have a shortened name, reuse the long name.
Publisher is the company that published your game (i.e. Atlus).
Title ID is a four-digit hexadecimal number (i.e. FB1D). This must be unique, as installing a game with a conflicting Title ID will overwrite any existing installed games with that same Title ID.
1. Copy the retroarch.cfg file from the resources folder to the romfs folder and open this copy in a text editor such as Notepad or Notepad++.
2. Use the editing tools (i.e. Edit > Find & Replace) to replace all instances of ROMNAME with the long name of your game.
3. Make any other edits you want or need to make to the configuration file at this time, then save and close it.
4. If you want to see the PlayStation boot logo when starting the game, open core-options.opt in the text editor and change pcsx_rearmed_show_bios_bootlogo to "enabled", then save and close it.
5. Copy your PlayStation 1 disc image (which must be in the CHD format) to the romfs folder and rename it to game.chd.
6. Press the Windows key and R key to open the Run dialog, then enter cmd to open the Command Prompt.
7. Navigate to the ps1-3ds-forwarder's resources folder. If it's on a different drive, enter the drive letter and a colon (i.e. D:) to navigate to it, then use the cd command to navigate to the folders (i.e. cd ps1-3ds-forwarder then cd resources).
8. Enter the following commands (I'm using Revelations: Persona as an example with a Title ID of FB1D), remember to adjust the paths to match the files on your PC:
bannertool.exe makesmdh -s "Persona 1" -l "Revelations: Persona" -p "Atlus" -i "D:\ps1-3ds-forwarder\assets\icon.png" -o icon.icn
bannertool.exe makebanner -i "D:\ps1-3ds-forwarder\assets\banner.png" -a "D:\ps1-3ds-forwarder\assets\ps1.wav" -o "banner.bnr"
3dstool.exe -cvtf romfs romfs.bin --romfs-dir "D:\ps1-3ds-forwarder\romfs"
makerom.exe -f cia -o "Revelations: Persona.cia" -rsf "D:\ps1-3ds-forwarder\resources\template.rsf" -exefslogo -elf "D:\ps1-3ds-forwarder\resources\retroarch_3ds.elf" -romfs romfs.bin -icon icon.icn -banner banner.bnr -DAPP_UNIQUE_ID=FB1D -DAPP_SYSTEM_MODE=64MB -DAPP_SYSTEM_MODE_EXT=124MB -DAPP_ENCRYPTED=false
9. Afterwards, you can clean up by deleting icon.icn, banner.bnr, and romfs.bin from the resources folder, and retroarch.cfg and game.chd from the romfs folder.
