I'll put it down in steps: 1. Build the map in question. Good Forge creations can take weeks or months. You can find a guide to advanced Forging techniques here. 2. Take screenshots of your map (in theater mode). 3. Come in here and start up a new topic in whichever sub category your map fits into. 4. Retrieve your screenshots from Haloscreenshots.net. Search your GamerTag, find the pictures you want, click on them, and copy the BB code at the bottom of the page. 5. Put info about your map into your post: How long it took you, what gametypes it plays with, the number of people recommended, a weapons list, etc. After that you should have a good map post. Please note however, that this part of the forum is for posting maps only and not information regarding them. If you have any more questions, or want a slightly more detailed version, just click on my name and sent me a Personal Message.
Read Hunters post. It's very well set out, unlike other guides saying 'uplode yor pics lol' This should probably be in the Halo Forge Discussion though. I can see you're extremely new, so you'll likely be let off.
Welcome to forge hub. If you find the other comments confusing click the link for another full guide on posting. How to post your map!! - Forge Hub Halo Forums