Creating an .osu file
Note
If you already have the .osu file you want to use, you can skip this section.
You start with creating the .osu file, which is done by using the Osu! Beatmap Editor. You can download it from: https://osu.ppy.sh/home/download
Osu! itself comes with a Beatmap Editor inside, allowing you to create your own beatmaps, or edit existing ones.
Upon launching the program, you can access the Beatmap Editor with the Edit button:
You can start by dragging an .mp3 file into the Osu! window:
Click on the new entry that was made for the song you just dragged.
First thing you need to set up is the Difficulty you intend this beatmap to be for (the OK button in order to proceed is disabled, until you choose a Difficulty for your beatmap).
If the mp3 file you dragged has proper Artist and Title metadata assigned, they’ll be automatically typed in for you.