Find the link url of the MAp  just like this

Download Dota map 6.75 Link

Download

1. Many of website using a blue url link.. click The blue or (any color with a link) word Download. And click save..

Next go to my Computer locate Folder

Then you copy the downloaded file into this directory:
ex: C:\Games\Warcraft3 TFT\maps\downloads

Paste it there or create a new folder.

And there you go you can use your map now.