Assuming you're on a firmware version 11.3 or lower, just follow
The Guide to the letter (use Homebrew Launcher - Soundhax). Once you're done with that process, simply follow the instructions for the Drag & Drop pack of Nova Sun / Umbra Moon in this thread! The mod will load over your real copy of Sun or Moon.
Any video-based guides for hacking your 3DS are outdated and install
arm9loaderhax instead of
boot9strap, which takes longer to install and is a riskier and more complex process. Follow the text-based guide instead.