Sketch a smooth curve that passes through or near specified points.
For example, click Annotate tab Detail panel
Detail panel
 (Detail Line).
 (Detail Line). 
 Draw panel
Draw panel
 (Spline), and place the cursor in the drawing area.
 (Spline), and place the cursor in the drawing area.  (Modify) to complete the spline.
 (Modify) to complete the spline. You cannot create a single closed loop with one spline. However, you can close the loop with a second spline. See Modifying a Spline.