Having house party in Shinjuku.

I would like to throw a houseparty in Shinjuku.

3 Korean guys are going to Tokyo in early July and will rent 3LDK apartment in Shinjuku area. Few years back, I was invited to a houseparty to 7/7 party. Every guests were wearing Yukata, had wine and did hanabi together.
I would like to throw a same kind of party however I have no idea where to start.

Please help me. lol

Sukiyaki would be a good dish to share at a house party. So just plan a sukiyaki dinner, invite some friends, then go to convenience store after supper to buy fireworks, and play with the fireworks in a safe location.

3 Korean guys are going to Tokyo in early July and will rent 3LDK apartment in Shinjuku area. Few years back, I was invited to a houseparty to 7/7 party. Every guests were wearing Yukata, had wine and did hanabi together.
I would like to throw a same kind of party however I have no idea where to start.

Sounds like you already have a pretty detailed plan. Exactly what do you need help with?

For a house party, go to the supermarket and buy the food and drinks you want, buy hanabi and arrange yukata (you can buy them at Uniqlo or department stores) if you want that to be a part of it- what is it you need help with? House parties here, as in any country, are casual gatherings, you basically do what you like, while trying not to annoy the neighbours.