How do you cause and reduce the soft spot on the top of a newborns head?

You don't. The soft spot or spots (front & back) represent areas where the edges of the skull bones are separated by more than a few millimeters. They can be as small as a pea at birth, enlarge over a few months & for most seem closed by 15-18 mo. Some start out big, i've seen one softball sized & had a helmet made to protect the head. Graduall closure over time is based on genetic factors, not your efforts.