What I always do:
First open game and go into Map Maker. In map name and map display name write same names (for example "telephone"). Then click export - it will create folder and every needed files.
Height Map:
Go into FeralHeart -> media -> terrains -> defalut. There's file named DefalutTerrain. Open is by GIMP or Photoshop and just draw on it (but only with grey colours). When you finish it, name it YourmapnameTerrain (for example TelephoneTerrain), save it as .png in FeralHeart -> media -> terrains -> yourmapnamefolder (for example telephone). Then go to Map Maker in-game and in map terrain name write what you named file. Click enter and it should work c: