1. Select Creative
2. Select Forge
3. Select [insert halo game here] (lets say Halo 4)
4. Select any random map available on the bottom row (lets say Adrift)
5. Select Forge game type, accept options.
6. Click Start, wait for the map to load.
7. Pause game.
8. Click Save As, click Yes
9. Name the map anything (map 1 of 5)
Repeat steps 7, 8, and 9 5 times in a row, the achievement should now pop.
If you are having trouble getting the achievement to unlock, restart Halo MCC & repeat steps 1 through 9, 5 different times, doing 1 restarts between each map save. The achievement should now unlock for you as well.