It is very easy. Use unassumingvenusaur's Gay Fates hack, if you don't want to use the Gay Fates hack, empty the fefates(birthright/revelation/special edition) folder first. Download what patch you want to, then copy the patch into the fefates(birthright/revelation/special edition) folder.
Be warned that you may need to copy files to outside from patch\rom folder of the patches zip file.
The fefates(birthright/revelation/special edition) folder directory structure should be:
fefates(birthright/revelation/special edition) folder
|-castle D
|-GameData D
|-m D
|-movie D
|-sound D
The fefates(birthright/revelation/special edition) folder should not contain any patch or rom folder.
Now launch Fates and in the NINTENDO 3DS splash screen the top screen should flash green momentarily just like other NTR plugins.
If done right, the character creation screen will have 5 voice selections.
One caveat, if you want to end Fire Emblem Fates with the patch on, always hold the power button, and switch off the 3ds first.
If not, the next app/game you launch will infinitely stuck in NINTENDO 3DS splash screen and only can be fixed by holding down the power button until the 3DS switches off. Don't worry if you got into the infinite stuck screen, no harm is done and your console is safe.