Instructions
Download an unlocked version of the "DotA" map that corresponds to your game version and patch. For example, use the website listed in the resources and check for the most recent update, which will be in the forums listed like this, "[Released]Unprotected Dota-Allstars 6.49" where 6.49 is the version number. Alternately, you may unlock the map by hand if you have extensive coding and decoding experience.
Open the downloaded map in the "Warcraft III World Editor" program that came installed with your original "Warcraft III" game. This is generally accessible in the same file directory as the one you installed the game in.
Edit your map using the tools provided in the editor. Save under a new, custom name when finished, making sure to save the new map into the directory with the rest of your maps.
Start "DotA" and launch the map that you created. Check carefully for any bugs or glitches before publishing online, if you choose to do so.