Add Geometry
It's time to pull out those measurements and pictures from the day at the shooting. If you have any objects that will somehow interact with your digital elements these have to be recreated in 3D. The complexity of the geometry depends on the situation but in most cases geometry will have to be added at least for the shadow casting information. Before you start modeling make sure you're aware of the units you are working in (window->settings/preferences->preferences->settings). Personally I prefer cm.
If you have the measurements and/or reference pictures from our scene you should try to re- create the objects as accurately as possible as these will be used as a reference point to align your camera later.
Aligning Camera
Here is an example where the added geometry is aligned with the background plate. You may have to adjust the cameras' "focal length" and "pre scale" settings to make the geometry align but make sure the camera-object distance is close to the real world settings or you may end up with some funky scaling issues. Turn on "objects details" (display->heads up display->objects details) to get the camera-object distance.
camera setting for aligning geometry
You may have to make minor adjustments to the geometry to make it a perfect fit (as you know the world is not perfect) but this is OK as long as you keep the general size and distance information. When this is done remember to lock the cameras' translation and rotation(select->right click->lock selected, should turn gray in "channel box")
Locked attributes turnes gray