How do I create a user-defined target/marker at runtime using augmented reality/Vuforia?

I've searched around for quite a bit for a tutorial on how to create user-defined marker in Vuforia, but haven't come up with any clear answer. Perhaps I've been looking too hard and overlooked something. I would really like a step by step of adding prefabs from the Vuforia package and how to implement them.

What I'm trying to do:

My project will be an Android based game where a user can take a picture of something through their camera (for instance , something on the wall) and then add a 3d model to it. This seems like it should be VERY easy to do and is the whole purpose for the Vuforia package, but I can only find tutorials on how to upload/download targets from the Vuforia website...not anything on run-time user defined targets.

**Also, I wouldn't mind a link/direction that explains the Vuforia scripts in detail for my own customization.

Thank you for any and replies.