TL;DR: just keep doing paizuri on her
I will assume that you already have Hoozuki in your room, and are asking how to get her ending.
It's the same as all the other girls. Every time you do paizuri with one of them, they drain some of your exp and their affection for you (heart level) goes up. You can check all their stats on the stats screen in the main menu. Once you get a girl to max hearts (which may require doing various side quests for them depending on the girl), you will have the option to propose 'Marriage' to them. Selecting this option will give you that girl's ending, and the game ends. If you choose to wait, and max out hearts for ALL girls, going to sleep will bring up a prompt asking you if you want to end the game. If you choose yes, you get the true (harem) ending and the game ends. *Note: you don't need Hoozuki (ghost girl) for the harem ending (because she doesn't even show up), but you DO need to max the fox girl you find in the tower.
Hoozuki isn't required for the true ending, but she gets her own ending just like everyone else. The only way to raise her affection is to do paizuri on her. So just keep doing paizuri until she gets max hearts, then choose 'Marriage'. Done.
So total endings = 9 girls + 1 true ending = 10 total endings. Hope this clears things up for anyone reading this.