Lets make a terrain called "HighFly" hehe!
1) Make a terrain called HighFly in the editor. One way to do this is by opening the curterr.cfg file with notepad and type in HighFly then close file while saving changes.
2) Start the editor select offline
editor
edit terrain
right side close
** do you want to save ** yes
You will now see HighFly in the terrains folder. (you may need to fresh explorer)
3) Goto Bmp2Map folder and copy "my_image" and paste it somewhere, rename it "HighFly" and move it back to the Bmp2Map folder. You can use this to draw in because it already is set to 1024 x 1024 grayscale. Unless you have another grayscale image that you can use.
4) Open your image with windows paint (that's all I have) or some other paint program and do anything to the image you want then save. Note! Pure Black gives the Lowest Alt and Pure White gives the Highest.