Therion's original idea was that it's no necesary to take LRUD, since you usually draw to-scale sketches in your survey book while surveying. These sketches do have LRUD data implicit, so it was (is) expected that you scan them, then load them into map editor as “Background images”. Then you draw over you own sketches.
From 0.3.7 version, Therion can show centerline with LRUD data in map editor window, so you don't need to have neither scanner nor to-scale sketches. Instead you have the cave's skeleton so you can draw your walls, etc, accordingly.
You can choose the method that best fits your needs, or combine both methods. This guide is focused to use LRUD data. To know how to work with scanned sketches, please read Wookey's guide and/or thbook.pdf