To use the output .CIA file, your 3DS have Custom Firmware (Luma3DS + boot9strap). Without CFW, the console will reject the signature on any non-official CIA. If you haven’t installed CFW yet, follow the official guide at 3ds.hacks.guide . This article assumes you have CFW installed and FBI running.
However, the actual conversion tool can run on either: .3ds to .cia converter
By following the guidelines and best practices outlined in this article, you'll be well on your way to becoming a .3ds to .cia conversion expert. Happy converting. To use the output