To download, just save any character card to your drive. So for instance the post directly above yours contains a card. I woul right click (using Chrome) and save image. It's a PNG image so I assume it is a character card although it may be a screenshot (later confirmed it is).
There are also plenty of other places such as the Discord server from which to download character cards.
Then move the file to your game drive in the subdirectory UserData/chara/female/
It should then be available to add to the game (assuming it is a character card). You can then adjust it using Character Maker if you like. See image (the card looks quite different from the image) but the image in loader matches that posted.
View attachment 2219315
To add it to an actual RG game. Load the game, then go to World Map, click on Resident Registration (middle icon on bottom left), then you can add characters.
View attachment 2219319
At this point I think who you can add where is determined by some of their Character Type.
The above card is set to a student so I think that is why she doesn't show up for an office. But I could add her to the Seminar. I haven't read anything official about this last bit. It is based on my limited experience.
Hope that makes sense. Best of luck