1. You need GIMP (Google and download it if you don't have it) Photoshop will also work if you have that.
2. Select "New" and set it to 513x513 pixels, RGB mode.
3. Paint it in with Red, Green, and Blue; Red will be texture 1, Green will be texture 2, Blue will be texture 3 in your maps.
4. Save it as either a .png or a .jpg (in my opinion, png is better)
5. Go into your map and in the load it into there.
6. Pick your first terrain texture and pick others by using the arrow above the terrain texture select for help.
(I can explain more detail if needed)