This is a very easy achievement.
On the main menu, go to customization, then landscape editor.
Create a level, then save it.
Now, go host a player match on Xbox LIVE, and when you get to the point of selecting the level, instead of randomly choosing, find the option to select one of your own.
Please note, according to people's comments, you can only select a custom map if it's a player match with your friends.
Play through the match completely and at the end, the achievement will pop!