To maximize extension of the thoracic spine and stabilize the inferior angle of the scapular, the optimum position is to place the users feet on a bench or chair, or for advanced users on a swiss ball. From this position, the ability to extend the lumbar spine is reduced. To lock off extension even more, have them push their heels down into the bench, activating their glutes and inhibiting lumbar extension. Any overhead work then creates extension in the thoracic spine, thus mobilizing it.

In the movements where the legs are fixed and the arms are moving, the mobilization is more on the upper thoracic. In the movements where the arms are fixed and the legs are moving, the mobilization is more on the lower thoracic.

Watch the following videos for some ideas:
Thoracic Mobilizer 1:

Thoracic Mobilizer 2:

Thoracic Mobilizer 3:

Thoracic Mobilizer 4: