Where to get the game?
https://sukebei.nyaa.si/view/3359691 (Highly recommended version, comes mostly updated where most cards should work right off the bat)

Official sites (Buy the game legally and support the development team)
https://dlsoft.dmm.co.jp/detail/illusion_0027pack/
https://dlshop.illu-member.jp/products/detail.php?product_id=201
https://store.steampowered.com/app/1073440/__Koikatsu_Party/

NijisanjiEN Koikatsu Cards
https://mega.nz/folder/xRtUQAQA#v5EXJtZE8qqt0b3H-d41Xw
Save the character cards inside your game folder at "\UserData\Chara\female or male, depending on the card"

You can find more cards in websites like pixiv by searching: "キャラ配布(コイカツ)"
And sites like:
https://illusioncards.booru.org/index.php?page=post&s=list&tags=koikatsu
https://db.bepis.io/

Opening the game you might want to start a new game, to unlock new positions and options in Free H mode
The scenario is a school of only girl where your goal is to gather members to your club.
If your using characters with Mod you will want to use a small number of them otherwise the game will load slowly or crash at times
Some small hints:
https://i.imgur.com/0aHur9G.png
https://i.imgur.com/P12n966.png

In free H mode you can use any of the characters made by the community as you want and have fun with them.
This mode is self explanatory but some small hints
https://i.imgur.com/gFqyexn.png

Interested in using character studio?
Update the game using the kk manager and look for updates
A small guide is how to use studio https://files.catbox.moe/smwwnx.pdf (not by me)
As for scenes created by the community you will need to save them on: "\UserData\Studio\scene"
Pixiv search: シーン配布(コイカツ!)