The easiest way to start is by installing Blender 2.73 (actually 2.71-2.73), and use Bitcopy's import/export script.
viewtopic.php?f=22&t=8086
This will let you load & save native model (and animation!) assets for LoG1 & 2.
(Bitcopy's script only works with these versions—no one has tinkered with updating it AFAIK)
LoG2 maps have a module height; found in the level's property menu. This sets the height of the level elevations. You may have noticed that the interior castle walls and ladders are of different height than their dungeon counterparts. You can make a wallset to suit your preferred module height.
Base set needs to have a pillar, floor, ceiling, wall, secret door (these are double sided), stairs_up, stairs_down, wall_text, secret button that matches your wall(s). Optional (but recommended) assets are: additional walls, additional secret buttons (small/medium/large); wall_drain, floor drain, wall decorations, door frames, light fixtures—certainly a torch-holder; portcullis, wall gratings, lever, locks (and the keys for those).
*You can have several variants of an asset, and have some of those distributed randomly in the tile definition; so that consecutive floors and walls need not be repeating copies of one another.
It helps a lot to simply load each of these into Blender, and customize; or just use them for visual aid. Load a floor model from the asset pack, and set your grid with it.
If you haven't done so already, then you will first want to download the official asset pack.
Optional additional tools: The GMT & GGAT editors by Dr. John Wordsworth.