Upon entering the Fortress of Doom in Doom Eternal, you have access to a variety of different upgrades and essential items locked behind sentinel-powered doors. One of the notable items in your inventory is the Classic Doom Armor appearance, which is on the first level behind the bridge.
To access this appearance option, you need to unlock it using two sentinel batteries. You can find these in any of the missions you deploy to in Doom: Eternal, or you can choose to complete the challenges scattered across every mission. There are three challenges per level, and if you complete all three then you receive a sentinel battery as a reward.
When you have at least two available batteries to use, take them to the back of your ship to the first level. The Classic Marine Armor is behind the door. To wear the armor, you need to back of your current game and return to the main menu. Go down to the customization options, and choose the Classic Marine Armor. Until your change the look, you will wear this appearance in all of your cutscenes.
There are a handful of other appearance options hidden throughout the Fortress of Doom. Each of them requires two free sentinel batteries to access, and you will need to return to the main menu to alter your option.