Master Forger Achievement in Halo: The Master Chief Collection

The achievement's description here is quite self-explanatory. From the Main Menu, enter Forge Mode and load up whichever map you want to tinker with. Once the Forge session has started, create one or two new objects on the map wherever you want. After doing this, save the map under the name and file description of your choosing. Now repeat this process four more times within the same Forge session and the achievement will unlock without any issues.