1. Go to the very first map of the game, where the player begins.
2. Click Publish
3. Type a description and select if it's done or not.
4. that's it, but if you have friends who don't use MGB, load your newly published game in the game player.
5. Select Embed link code or whatever, then copy that link and send it to your buds.
You find the publish button in the map maker by the way.
Hope I helped! =)