There's not much to it but here's a step-by-step:
• Find a 4-door car. After the prologue and as soon 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 is a callback to the achievement below, which is fondly remembered by The Division 1 veterans as one of the very first unlocks, celebrating the release of the Warlords of New York expansion that adds a new playable area in Lower Manhattan.
You don't need to own the DLC to unlock it. I had it pop retroactively for me as soon as my character loaded, and i didn't have the expansion installed yet.
Edit: Just in case, here's a gameclip showing how to accomplish this arduous task: