Featureless terrain or water - now there's a challenge. Is this from point of origin to point of arrival or only over part of the route.
You will still be flying along your chinagraph pencil line but have no ground features to refer to. You will still be able to establish the expected position on the line form elapsed time. So you will then need to get a fix using VOR/ADF which will give you an approximate position which you can assess relative to you original track.
If you're making long flights over water or desert the a GPS would be advisable.