- 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.
- Check the 'Closed' check box in the options tool bar if you want to create
  a closed spline. Closed splines are continuous closed loops.
- 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):
