You install the "NDS Forwarder Generator" homebrew app. You then navigate to your .nds files on your SD card, and the app "converts" them into shortcuts on your Home Menu.
Instead of a traditional converter, users utilize —small CIA files that act as shortcuts to launch NDS files from your 3DS Home Menu. Below is the definitive guide to using these modern "converters" and why they are the preferred method. Why You Can't Simply "Convert" NDS to CIA nds to cia converter
An NDS to CIA converter is a tool that allows users to convert NDS ROMs (games or applications originally designed for the Nintendo DS) into CIA files, which can be installed and run on the Nintendo 3DS using homebrew tools or the console's built-in software. You install the "NDS Forwarder Generator" homebrew app
Keep in mind that the use of NDS to CIA converters may be subject to specific regulations and guidelines, depending on your region and the intended use. Below is the definitive guide to using these
For further details, consult the official documentation of Forwarder3-DS or the GBAtemp forums.