Workflow
The basic premise here is to use the Osu! Beatmap Editor since it has a robust set of tools for authoring and playtesting beatmaps, then to bring the resulting .osu file(s) into Unity.
Note
This only explains the basics on setting up your osu files. The art of placing coherent Hit Objects and making them harmonious with the song is beyond the scope of this manual.
There are many Youtube tutorials regarding basic controls, techniques, and the thought processes behind how a beatmap should be designed.
Youtube channel pishifat is a good example. Also, here is the documentation on the Beatmap Editor.