Simply copy the downloaded .png or .dat file into the appropriate folder. Do not rename the file to something random, as the game reads the internal ID; however, you can rename character cards freely—the preview image will update.
AI Shoujo save files are the lifeblood of the game's community. Whether you need a shortcut past the survival grind or a perfect replica of your favorite anime heroine, learning to manage these files will double your enjoyment of the game. Just remember to install the necessary mods first ai shoujo save file