Download the Better Repack torrent.
https://sukebei.nyaa.si/view/3934925
Extract the archive whereever. Run InitSettings.exe to set things up.
Run Koikatu.exe once just to populate config files.
Run KK Manager \Koikatsu[UTILITY] KKManager\KKManager.exe. Click "Look for updates". Let it cook, it'll come up with a list of mods to download. It will take awhile, you can skip "Studio" mods if you aren't trying to take fancy screenshots for Twitter and just want to fuck your waifu.

Female cards go into \UserData\chara\femaleFemale cards that you edit to have a cock go into \UserData\chara\maleClothes cards go into \UserData\chara\coordinateRemember that you must download the full version of the image, any websites that downsizes or turns it into a thumbnail are going to fuck up the metadata. Sites like 4chan remove metadata so you can't upload a card as an image post. Most people use catbox.

It's good practice that you try opening new cards you download in the character editor, just to check if they work. If you open them up and you see a bunch of errors about mods missing (and you haven't gone a year without updating with KKManager) then it's likely the card author made their own mods for the character. You'll have to ask around or find where they released it.
Manually downloaded mods go into \Koikatsu\mods. They usually take the form of a .zipmod file. Placing them anywhere in the folder is fine, the sub-folders are mainly for organization purposes.

/vt/horny/ might help you if you run into issues but they may also tell you to fuck off to /h/ if you're asking too much about troubleshooting. Pretty sure /h/ has a koikatsu general going around the clock.
Good luck, enjoy polishing your pebble.