As mentioned above.
To make a mask for your map, simply just go into the watermill folder located in feralheart\media\terrains. Copy the watermillmask.png and paste it into your map folder. Change the name on the picture to (yourmapname)mask.png. And save it.
Once you have it you edit it around with Gimp or PS. it's RGB so Red is one texture, green is another and blue is one. Inside the map maker under the world tab you set which colour that has which texture.
For making completely "own" made objects you'd need to learn to mesh in a program suggested above. Though you can create own objects using the "Object maker" inside the tools menu from the starting menu. This lets you tilt, change sizes etc on already existing objects inside the game, then save it as a new one. Lets say you'd want to make a pyramid out of rocks for some reason, you can use the object maker, and stack the rocks there, save it as an object to a new object group. The next time you open your map maker, you will have a new group there with the object in it, and already a made pyramid.
Learn by doing really.