Well, there's not much to it but here's a step-by-step:
• Find a 4-door car. After the prologue and as soons as you regain control in Washington D.C, they're everywhere.
• Examine one of it's sides. See if there's two semi-open doors. Most cars do.
• Latch onto it as cover by pressing
• Move from one side to the other till your character closes both doors. You can go from left to right or vice-versa, it doesn't matter.
This achievement is a callback to the one below, which is fondly remembered by The Division 1 veterans as one of the very first achievements most players unlocked there, celebrating the release of the Warlords of New York expansion that adds a new playable area in Lower Manhattan as a throwback to the first game.
However you don't need to own the DLC to get this one, since it's a title update. I had it unlock retroactively for me as soon as my character loaded, and i don't have WONY installed yet.Edit
: Just in case, here's a gameclip showing how to accomplish this arduous task: