Start with FEBuilderGBA. You can edit a unit's stats, change a portrait, and have a working hack in 10 minutes.
Underrated gem.
(Base: FE7)
The result is an almost endless library of "new" Fire Emblem games, completely free.
These represent the pinnacle of the scene—complete, polished, and unforgettable. Fire Emblem Gba Rom Hacks Download
A: Yes, using a flash cart like the EverDrive GBA or EZ-Flash Omega . Patch the ROM, copy the .gba to your SD card, and play on real hardware.
This is a unique entry. It is a "demake" of Fire Emblem Echoes: Shadows of Valentia (the 3DS remake of Gaiden). The creators painstakingly ported the 3DS game into the GBA engine. Start with FEBuilderGBA
The modern masterpiece.