First, connect your source footage to VolumeRays img input. The gizmo's default settings will generate the volume rays only, and through a luma key ('RGB Luminance' in the Volumetric Creation pull-down menu). Second, move around your vol_pos point in the viewer so that it's somewhere behind your windows. Third, add this output to your original footage with a merge node, operation 'plus'. Alternatively, you can check the "Add on Top" box under the Volume Options section to see the effect added to the source footage internally.
If you have a matte for the windows, use a copy node to merge it into the stream just above the VolumeRays node and change the Volumetric Creation pull-down accordingly.
Studying VolumeRays' internals could be a good learning exercise. Click 'VolumeRays' tab in the parameters window (located to right of 'User' tab), then click 'copy to group' button. Select the resulting group in the DAG, type ctrl enter to open group and poke around in there.