To add onto everything already said, Nightmare and FEditor shouldn’t be used since Nightmare can’t do everything, so you need to open FEditor, and FEditor is widely known for breaking stuff and generally being a shit.
The two methods of hacking used now are FEBuilder and buildfiles.
FEBuilder is basically if you put Nightmare, FEditor, and everything else you need to hack into one app. You want to insert music? FEBuilder can do that. You want to insert portraits? FEBuilder can also do that? You want to write events for a chapter? FEBuilder can also do that. And, unlike FEditor, it probably(and by probably I mean 99.9% of the time) won’t eat your rom.
Good tutorials to learn how to use it are Shindad’s(or shin19 i dunno which username they prefer) FEBuilder primer as a way to familiarize yourself with the basic parts of hacking and Markyjoe’s Creating Chapters in FEBuilderGBA video tutorial which explains how to do a more advanced part of hacking, events, which basically control what happens in a chapter, like text and where units are loaded/move to during cutscenes.
Buildfiles work differently to FEBuilder and FEditor/Nightmare. I honestly don’t really know myself how to explain it(because I myself am a noob) but there is a tutorial for it that can probably explain it better. Generally this is less newcomer-friendly than FEBuilder, but ultimately it is better for more advanced projects as you get better at hacking.
There are also a few other applications you might need for hacking. Usenti is great for making portraits do to its grid and requantize so you don’t use too many colors, while Tiled is great for making maps(Edit: I forgot FEBuilder has a mapmaker built in that works just as well as Tiled), and Event Assembler is a must have for hacking in general. I think all these programs can be used on mac through wine?
Any other programs you might ever need for hacking can be found in The Ultimate FEGBA Resouce Repo.
There are also quite a few other good tutorials-Blazer’s original Ultimate Tutorial is great for the basics of hacking, even if the methods used are outdated by now, Markyjoe’s Tiled tutorial shows how to use Tiled, while Pikmin1211’s guide to map design is great to show what a good map should be like, Toaomr’s spriting tutorial shows how to make a decent portrait without needing to actually art too much. There are countless other good tutorials you can find on the Tutorials category here on FEU.
While FE7 can still be used as a base, most stuff right now is being developed for FE8, making FE8 generally better. That said as long as you aren’t hacking FE6(and you aren’t Onmi) you should be fine.
Finally, it’s generally good to try and search both FEU and google if you run into any problems. If you have a problem, there’s a good chance someone else ran into the same problem or made a tutorial to solve that problem.
Good luck on your hack!