Just draw some random shape pressing the space bar as you wish to add dots in the way (dont cross lines), be sure to join the last vertice with the first one. then, move around the narrow pressing the mouse´s right button, once the narrow is inside the sector (all the space within the drawing is a sector) press the little enter of the numpad. If you could follow these first instructions okay, you should be now in the 3d mode, aim the walls and floor/ceiling with the cursor, pressing V twice will bring you access to the tiles set, once you choose a proper tile and press enter, this will get printed over the surface you was aiming to at the moment of pressing V. Finally, when you got a good textures combination, press pgup and pgdown to move up and down ceiling and floor.
That´s just the very basic begining, but as you see, it´s not something for wizards.