you dont really have to since you can just extract it and drag that folder into the Skins folder in the osu directory, but if you want to, make sure with Windows Explorer you have "File Name Extensions" enabled, then right click the .zip and change the .zip to .osk, that should work
Skrublord, I play osu! Hopefully by the time you read this ill be able to play 5 star maps, or I could still be horse shit FeelsBadMan
Edit: I beat a 5 star, now I need to beat a 6 star, and get past 1k
Edit2: I got 1k PP, now all I need to do is beat a 6 star hell yeah
Edit3: 103PP PLAY LETS FUCKIING GOO, have yet to beat a 6 star
Edit4: I beat a 6 star and a 7 star, im also at 3.1k pp wooo only need to be a 8 star now