Once you begin sketching the first profile, it can be useful (although not required) to locate the profile with respect to the origin of the Work Plane.
When a new model file is started, the Work Plane is automatically oriented with its origin and axis corresponding to the Absolute (World) coordinate system. If you move the Work Plane, you can right-click on the Work Plane border or origin and choose On Absolute Reference System (World) to bring it back to the World reference system.
The starting point of the profile can be at the World reference system origin. As you draw the profile, you can snap to the Work Plane origin and the program automatically adds a Coincidence Constraint to that point. In order to lock another appropriate point on the profile, use the Ground Constraint, which can be applied on an entity vertex or arc center point at the desired location. Features added later should be located with respect to existing model geometry, in order to propagate model changes easily.
If you are inserting a Ground Constraint on an existing profile, then first start its editing mode and thereafter, insert the Ground Constraint at the desired point.