Now I will tell you how to add a map.
We go to the official site osu.ppy.sh and click on the "maps"
Find the right song for the map with the help of a search engine
To download you need to go to your profile and click on the "download" button (in the lower right corner)
№ 1
№ 2
You downloaded the map and now you need to open it in osu! (It is advisable to enable the function "open all files of this type in osu!")
№ 3
I hope my advice will be useful to you. If you have any questions, write.
With best regards, Alex