You are looking for two different things, you cannot (and should not) blend them into one.
Koikatsu, the main game, allows you to create character cards and that is pretty straight-forward, you should be able to do that after some playing around.
CharaStudio is the main software you will be using in order to pose and animate characters, set the various cameras and build the background... Even import new models if you get your hands on Unity 5.6.
You can even use 2D images as background, with some perspective work.
Here is a small guide to get you started:
You must be registered to see the links
the rest comes with practice and trying out new things.
Once you are done with those two things, assuming you downloaded all the necessary mods or got the game already pre-modded, you should be able to use two hotkeys in character studio: "shift + r" to record an animation in a sequence of pngs and F11, in order to take a still frame (screenshot) depending on the plugin settings (Default is 1920x1080, I believe).
You can turn those still frames into an animation by using software such as FFmpeg.
The second part of your question, "How to use them on Ren'Py" is another topic entirely.
Koikatsu simply allows you to create resources for your game, nothing more.
To be able to use said resources on Ren'Py, you need to learn it by using a wide variety of tutorials available online, the engine itself come with two tutorial projects.
