Click the Curve ► Spline Curve command 
in the Geometry
menu.
Enter a Tolerance value if required. The smaller the tolerance value, the smoother the curve appears on the screen. As the curve is mathematically defined, changing this modifier does not alter the actual shape of the curve.
If required, enter the slope (in degrees) at the start of the curve into the Start Angle. To take the angle of the slope from an existing entity, set Start Angle to <Digitise>.
If required, type the slope (degrees) at the end of the curve into the End Angle box. To take the angle of the slope from an existing entity, set Start Angle to <Digitise>.
Choose a Break Points option. This can be either Uniform or Chord Length (see About Splines for details).
Type a Name for the curve, if required.
Digitise the fit points for the curve. The fit points can be full 3D points.