Instructions
Open "Warcraft III: Reign of Chaos" by clicking the program in the "Warcraft III" folder in the "Start" menu. The game will automatically boot to a blank map.
Create a new map file that meets your preference for the size of the level. To do this, click "New map" from the "File" menu on the top toolbar. A window will appear allowing you to edit the preferences of the map. You can always change the settings later.
Create a map using the program's editing tools. The editing toolbar is located on the left-side of the screen and has a plethora of tools. Start by laying some terrain on the ground then adding units and other features.
Save the file by left-clicking on "Save" from the "File" menu on the top toolbar. A standard saving menu will appear. Give the file a name and click "Save." The file will be ready for use and will be unprotected.