just follow the steps
1st extrude a simple line into surface
than select freeform feature from the surface tab and select the face you want to deform
now click on add curves.. their is two type of options through points and control polygons.these curves act as control curves
here is the simple horizontal control curve using "through points" option
to add vertical curves press "TAB"
Now click on add points
And add points where ever you want on control curves
when you are done deselect everything and click on any point you want to deform
their are three types of coordinate system available to deform that point.. i prefer "curves" but its all depend upon you model and shape you required
when you are done click "Ok".
here is the deform surface