For windows:
Go to the FBI thread, get the latest version, and get sockfile (it's on the same page)
Install the latest FBI and run it (it has a blue menu)
Then run FBI, choose network install. It should tell you the IP adress.
After extracting sockfile, put the .cia in the same file as the sockfile_2.0.jar or whatever it says. I'd reccomend renaming the CIA to something short.
Rename sockfile_2.0.jar or whatever it says to sockfile.jar (unless you don't have file extensions on your computer, then just rename it to sockfile)
Make sure you are on the same internet as your3ds
Shift right click on an empty space in the directory, and choose the open command prompt option.
In the console, type,
without the quotations or brackets, "java -jar sockfile.jar [ip adress] [cia]"
You do not need the port.
This takes a while, 1.7 gigabytes took me about 2 hours.
It also failed the first time (idk why, maybe my wifi went down for a minute)