You can load the chapter and level from the main menu if you missed it. I did after I finished the game.
Load up Chapter 4 - Vimy Ridge.
Right near the start, after you fire the first rocket at a pillbox, you'll follow the commander through gunfire from two pillboxes and an airplane. Then you'll come another rocket, this one you have to adjust to match two German artillery units shooting at you and your men. This is where the German flag is; between the two artillery positions.
The left position is at 60
The flag is at 35
The right position is at 10
You should see the flag being taken out from the position you actually fire the rocket from, and it should pop within five seconds of it being shot.
Edit: If you're having trouble getting the achievement to pop:
"I've tried it now 10-15 times and nothing.
Then I just closed the game completely and restarted it, tired it again and blop; first attempt." - II W I K U II
(I'd like more feedback on this to see if it was just a lucky one-off, or if it does seem a viable solution for people it isn't popping for)