How do you visit someone's forged map?

i’ve seen on youtube someone went to somebody’s map and played there.

can you tell me how to do that

If you know the creator’s gamertag, you could search for their File Share and see if they have it available for download.

I’m going to assume you aren’t a troll. As the Admin stated is one way, but did the person include a link to the map in his fileshare on the youtube video? If he did, you can just click that. Another way is to do a File Search on B.net or in game, using the creator’s GT, the map name or relevant tags. Lastly, if you have played on the map and want to keep it, you can access it through your temporary history.