Adding notification sounds and ringtones to the iPhone

Hello,
I have an iPhone 14 pro, running Ios 18.3 Beta 2.

I would like to add some notification sounds and ringtones to my iPhone, but don’t know how to go about it.

The files I have are in mp3 format. In 2020, when I got my first IPhone, a Se 2020, I tried and only succeeded to add two files, but both resulted into ringtones, while I wanted one of them to be a notification.

I had to use Itunes, or Imaging, but I was told this should no longer be necessary.

I’D be extremely grateful to anyone who could give me some instructions on how to do this, provided it is possible.

Thanks in advance and, since this is my first post of 2025, Happy New Year to all! Ollie