I simply downloaded UnRarX
Its a website that convert .rar files
once you downloaded UnRarX click on it and something will pop up.
Click okay then a box will show
Drag your .rar file into the white box and it will 'unpackage' the things in it into a folder
Then drag that folder to the skins folder in Osu!
Hope this helps
Link to UnRarX:http://www.unrarx.com/
Also Video Link That might help:https://www.youtube.com/watch?v=EvEYq7odwKM
Note: This Is Not My Video
Its a website that convert .rar files
once you downloaded UnRarX click on it and something will pop up.
Click okay then a box will show
Drag your .rar file into the white box and it will 'unpackage' the things in it into a folder
Then drag that folder to the skins folder in Osu!
Hope this helps

Link to UnRarX:http://www.unrarx.com/
Also Video Link That might help:https://www.youtube.com/watch?v=EvEYq7odwKM
Note: This Is Not My Video