If you are drawing on a vector layer, the Brush Properties dialog will have a Smoothing tab, which allows to configure how Storyboard Pro automatically smooths your brush strokes. If you are drawing on a bitmap layer, this topic is not applicable, as bitmap layers do not support automatic smoothing.
In the Brush Properties dialog, open the Smoothing tab.
Use the Centerline Smoothing slider to set the amount of smoothing Storyboard Pro should perform on your stroke's direction, curves and corners. A higher setting will prevent your line from being shaky, but is liable to reduce your drawing precision and to round sharp corners into curves.
Use the Contour Smoothing slider to set the amount of smoothing Storyboard Pro should perform on your stroke's contour. A higher setting will round sharp corners and reduce the amount of control points for the resulting vector shape.
In the Stage view, start drawing to test your brush settings.