All the tools that you are going to be using for terrain editing utilize the same set of default brushes. The number of brushes is limited, as such so are our options; however, we can also import our own custom brushes and use them.
For this exercise we will use GIMP 2.8 to create a brush, but you can do it with any 2D app of your own preference.
First, we will create a new file; it doesn't have to be large—64 x 64 will fit just fine. If you can, you should specify that you'll be using grayscale colors and set a transparent background (Photoshop users can simply change to the grayscale mode and delete the background layer).
Using grayscale colors, draw whatever you feel like. Remember, the darker the value the stronger the ...