Now, put FBI.cia in your cias folder If you do not have this folder then create it
Now, put FBI.3dsx in your 3ds folder
Insert your (micro)SD into your console
Section II - Installing FBI (FBI.3dsx)
Turn your console on
Enter The Homebrew Launcher
If you have The Homebrew Launcher app on the home screen, use that
If you do not have The Homebrew Launcher;
Open Download play or Health and Safety information, Open rosalina menu (usually L+Down+SELECT) Go to Miscellaneous options, then click on Switch the hb. title to the current app and restart Health and Safety information / Download Play
Launch FBI
Open SD
Open the cias folder
Select FBI.cia
Select Install and Delete CIA
What you need
Universal Updater
Working Internet connection
Section I - Installing FBI (UU)
Open Universal Updater
Search for FBI
Press A while hovering over FBI from the list of homebrew