I was given wireframes of the scenes from the CG animators which I then painted in Photoshop according to the project style. Below you can see an example of a wireframe and then the finished background:
The images are created at a size that allows for camera moves within the shots. For example, the image below is wider than the TV ratio because the camera move goes from left to right within the image. I also had to draw more information on layers so there was enough imagery as the camera moves around revealing different parts of the picture.
I was also given matte references for the 3D objects to help isolate areas for colouring:
Additional shadow layers were added by the CG programme (Maya) into footage that was then composited together and colour graded for the final output. Here is the finished commercial:
Here is a time lapse video of a background I made for the most recent commercial: