Hi Xavier!

There are two methods that will allow you to draw on top of a surface:

• The first one is called Sketch, type _Sketch in the command prompt hit enter and then from the options select OnSurface, select a surface and start drawing away. The command is also located under Curve > Free Form > Sketch on Surface.
• The second one is called Interpolate on Surface, type _InterpCrvOnSrf in the command prompt, hit enter, select a surface and draw away. The command is also located under Curve > Free Form > Interpolate on Surface

The Sketch method is more "sketchy" as the name implies and is more loose and less accurate. The Interpolate on Surface is more accurate as it let's you select the control points which you want your curve to pass through. Try them both and see the difference.

Good luck!

Arie