Read
https://3ds.guide, you can install b9s from the homebrew launcher.
1. Uninstall theme hax by booting into your sysNAND (eg turn on the console with the SD card out then put it back in when the home menu loads) and changing the theme colour. If you don't do this, after you install B9S you will get a black screen on boot or an error on boot and the only way to fix it will be to remove theme hax manually by deleting files from the Nintendo 3DS folder.
2. Follow
https://3ds.guide/installing-boot9strap-(soundhax) but you don't have to actually use SoundHax if you have your own preferred method of getting into homebrew
3. While following the instructions there will be an optional instruction to move emuNAND, follow that