Fang, Jihl, Lebreau, Sara, Vanille, and of course Lightning will be walking around the city like normal, such that you can interact with them, run them over, shoot them, or whatever comes to mind.
All you have to do to install the mod is open the file "pc/models/cdimages/componentpeds.img" with the help of SparkIV, and replace the files with those that come in the compressed file. It's that simple. Within seconds you'll be enjoying the mod.
GTAIV: Final Fantasy XIII Girls Pack is one of those mods that while it doesn't add anything really useful to the game, such as things like new cars, guns, or better graphics, it is interesting enough (especially for Final Fantasy lovers) to try out.