contour lines

Problem with v.digit and contour lines

Hello,

I want to make a digital elevation map using a topographic map as a
backdrop cell map.

But when I use the menu "label contours" I get the message "not correct
number of intervalls". I think I tried every possible combination of
digitized arcs and the number of contour intervalls, so it seems that I
didnt`t understood a very essential part of this feature.

If somebody could explain to me step by step how to do this, I would be
very pleased.

Thank you,

                                                        Fellini