Since you cannot buy this on the eShop, the installation process differs slightly from standard games:
: Note that original RPG Maker MV games (like the modern Omori release) are very demanding; the version playable on 3DS is typically a de-make or a specifically optimized port for the RPG Maker 2003 engine. or instructions for transferring game data to your SD card? 3DS:Installing software - Hacks Guide Wiki omori 3ds cia install
To install any .cia file (which stands for TR I mportable A rchive) on a 3DS, the console must have Custom Firmware (CFW) installed. The industry standard for this is currently Luma3DS . Since you cannot buy this on the eShop,
: Power off your 3DS and insert its SD card into your PC. Create a folder named cias (or similar) on the root of the card and copy your .CIA file into it. The industry standard for this is currently Luma3DS
Do not use YouTube video guides for modding (they go out of date). Use the definitive text guide:
Canceled. The 3DS stretch goal from the original Kickstarter was replaced by the Nintendo Switch port .
: A high-capacity SD card (formatted to FAT32) is recommended, as game files can range from 200MB to over 1GB depending on the version. Step-by-Step Installation Guide