1. Choose the degree of the spline in the options tool bar. Supported degrees are 2 (quadratic b-spline) and 3 (cubic b-spline). The higher the degree, the 'smoother' the curve becomes.
   Note that a quadratic b-spline requires by definition at least 3 control points while a cubic b-spline has at least 4 control points.
2. Check the 'Closed' check box in the options tool bar if you want to create a closed spline. Closed splines are continuous closed loops.
3. Specify the control points. After defining the third (for quadratic b-splines) or fourth (for cubic b-splines) control point, a preview will be shown. You can remove the last control point again by clicking the 'Undo' button in the options tool bar.

Example for two open splines (quadratic and cubic):
Example for two closed, periodic splines (quadratic and cubic):