How do I use a thumbnail for my forge map?

Games Halo 5: Guardians
1

I recently just finished my first forged map. It’s about ready for customs, I just don’t know how to create my own thumnail similar to ‘Don’t Breath’, ‘Survive the city’’, etc. Please help

2

Go into theater mode, and take a cool screenshot of your map by double-tapping the guide buttonn and pressing Y. Then, go to the options for your map. Link that screenshot to the map. Boom.