Since it was asked, I will make a step by step guide to how to post a map.
HOW TO POST A MAP IN THE MAP ROOM.
Step 1: When you think your map is finished, by spawns, playablity and weapons. You can post it up.
Step 2: When you're in Halo: Reach, press start on the controller then go to filebrowser section and got to local files. In there you will find your map, click X and a window will pop up, click on 'add to fileshare'. This wont take long for it to be done.
Step 3: Go to custom games or forge, start your map and play. Then finish the game and go to Theater and take screen shots by pausing the film and clicking on the camera button.
Step 4: Next go to www.bungie.net and sign in, after that go to your profile by clicking on the Reach noble team logo. Then go down to 'recent screenshots' and upload the pictures you want to photobucket or other picture sharing sites.
Step 5: Go and make a topic in the map room, then click on the image button and copy the URL of your image over (found in photobucket). To get the download link, go to www.bungie.net and go to your file share and just go to your map and copy the URL. Then your all set, just write what you want and post away.
Step 6: Enjoy