What you need
- The .cia file from a dumped cartridge (https://wiki.hacks.guide/wiki/3DS:Dump_titles_and_game_cartridges)
- The boot9.bin and movable.sed of the 3DS system you want the games installed to
- The latest release of custom-install-standalone (https://github.com/ihaveamac/custom-install/releases)
- The latest release of custom-install-finalize
- Seed.bin (direct download) https://github.com/ihaveamac/3DS-rom-tools/raw/master/seeddb/seeddb.bin