Tiles
Let's begin with the most important thing: Tiles.
Tiles are the main building blocks of the map. There are two types of tiles: floors and walls. They use different textures (though you can import the same texture under different types).
Tiles import is done by selecting "Import Textures" in "Import Resources" menu:
Under each table there are two buttons: the left one will remove current selected texture and the right one will open a file prompt, letting you select one or more textures to add.
After textures were added, press "Accept" and all your added tiles will be shown in "Tiles" tab.