Getting a lot of comments about this achievement unlocking after several more maps than 25 rated. The impression I am getting is it will unlock, it just may take longer than expected.
Download 25 user made maps. Create a custom game. Go to the maplist screen and go to the bottom. Add 15 of the maps you downloaded (15 is the maplist limit). Now you can either invite friends and play the maps or do a private match and get a feel for them. If you are in a hurry, you can go to the pause screen and start a vote to end the match. On the results screen you should see on the lower right a thing that says hit Y to rate map. Do so.
P.S. If you are playing by yourself or are rushing through by voting to end the match, at least explore the map and get a feel as to whether it is a quality map or not. Not really playing a map is going to skew your view of whether it is good or not; not even getting a remote feeling for it will be worse. It is really uncool to vote on the quality of a map if you know nothing about it.