Step 1: Download the following tools:
Terrain 1.0
WinMPQ
Step 2: Create a map with the dimensions you want, with deep water level and LordaeronSummer tileset. It is very important that it is Lordaeron Summer, otherwise it will not work (you can change it later).
Step 3: Create an image wich has the same dimensions as the map, except +1 on each axis. So a 256x256 map needs a heightmap with the dimensions 257x257. Make the background black, and start painting with grey on it. Here are some examples of how it might look:
Step 4: Save the image as .tga (using photoshop, gimp, or similar). Open the Heightmap tool you downloaded, and enter the paths of your map and .tga file. Save the converted file as war3map.w3e.
Step 5: Open your map using WinMPQ. If you cannot find the map when browsing, make sure to set "file formats" to "all". Find a file named war3map.w3e and remove it, then import the new file you just generated. Close the MPQ editor and try opening the map in the editor. Your terrain should now be shaped after your image. Edges might be a bit rough though, so you want to sweep over them with the smooth tool a couple of times. Try tweaking the brightness and contrast of the heightmap until you have one which works well. After the heightmap is properly imported, it is safe to change the tileset.
IMPORTANT NOTE: As heightmaps can make the terrain angles surpass the maximum allowed by the editor, you need to use the MiscData hack to remove limits on cliff steepness. Otherwise the editor will crash when you try to edit the terrain where it is too steep. You can find the file attached to the bottom of
this thread, and paste it into C:\Program Files\Warcraft III\UI. If the UI folder does not exist in your wc3 directory, you can just create it. Happy terraining.