I was looking for a way to run NeoGeo games on my o3ds, and with Retroarch the games ran terrible, about 20 fps was the most i could get. I found out that using NeoDS, the DS NeoGeo emulator, you can play most games at a stable 60 fps.

For the purpose of this guide I will be using TWLoader, but any other way that lets you run DS homebrew should work. I've only tested it on my a9lh o3DSXl, running Luma 6.6, firm 11.2.

*Note: this guide is heavily based on this guide, which was used for use with a DS flashcart. I wanted to use it without a flashcart, and that's why i made this guide.

1. Download TWLoader and unzip it. Copy the .cia and the contents of sdroot to the root of your sd card.

2. Install TWLoader.cia, and under _nds/twloader/cia install TWlapp.cia using FBI or any other CIA installer.

3. Download NeoDS and uzip it. Copy NeoDs.nds to the nds folder (not _nds) and create a new folder on the root of your SD card with the name neogeo. Copy the contents of the NeoDS folder inside the archive to it, except the NeoDsConvert folder and the readme file. Copy NeoDsConvert to your pc.

4. Now we need to convert your NeoGeo roms. First we need a NeoGeo bios file. For legal reasons i cant provide it here. it should be named neogeo.zip and contain a few files. Copy the zip to NeoDsConvert on your pc, but don't unzip it.

5. Now we need a ROM file. Get your rom as MAME rom, and copy it to NeoDsConvert. For example, Metal Slug will be mslug.zip.
Open the zip, and check if all of the files are .bin. If not, we need to change that. Unzip the file, change the extension and zip it back with the same name, make sure its a zip and not a rar/7zip or any other file.
*If you're planning on playing only one or two roms manually changing them is fine, but annoying. I've used Extension Changer which lets you change all of the files at once, by dragging and dropping the folder on to the window.

6. Run NeoDsConvert.exe, and you should see a Command window come up and when its done two new files will appear in your folder, and the extension .neo. That means it finished and everything worked. If we continue with the Metal Slug example, it will be now called mslug.neo

7. Copy the Converted rom onto the root of your SD card, run TWLoader and launch NeoDS. Choose the game from the list and press start to run it. The game should now run, enjoy!

Please note, I've tested a few games and they all seem to work very well aside from a few very minor graphical glitches. This seems to be a limitation of the emulator, but it still works better than the Retroarch one.