Hey! So, I said I might make a list of Fire Emblem modding links, so here it is! Before anything, be sure you have a hacked 3DS so you can mod your games. You can load mods directly onto a hacked 3DS OR you can use a hacked 3DS to dump your games and play them on an emulator.
If you haven't hacked your 3DS yet, follow this guide to do so! It's easy and super safe!
As for if you want to emulate, you can download the citra emulator here. Just be aware that the emulator can be demanding and not all games function great (all the Fire Emblem games are functional, though).
Now, as for game modding, here are a few of my favorite things to use as well as some interesting mods others enjoy. Feel free to share mods you enjoy and I'll add them to this list.