Things you will need:
- Kuriimu: https://github.com/IcySon55/Kuriimu/releases
- A romFS of nsmb2.

Tutorial:
Step 1: Once Kuriimu has downloaded, extract it and open the 'Kukkii' application file.
Step 2: Once Kukkii is open, you will need to hover over 'File' and click 'Open'.



Step 3: A file manager window will pop up, what you want to do is navigate to your romfs, go to /Model/Demo/ and then select 'title_logo' (if you are playing gold edition it will be 'title_logo_ge' instead).

Step 4: You will now have 3 (4 if you're on gold edition) pictures like this:


(Mine looks different as I have already edited my logo.)
Now you want to click the 'import image' button on the top left and replace the 3-4 files with your own logo.

Step 5: Once you are done, press the save button on the top left.
You're done! Hooray! I hope you found this tutorial helpful, give me feedback if you have any issues. This is my first tutorial on this game so do tell me if it's alright!

I have quite a few problems with ohana3ds, this tutorial is probably simpler.