Make sure you and 5 other people are ready and willing to boost. Play a Ranked match on the Manor House level, making sure the game type is Capture the Flag with 5 flags to win and 30 min match (30 mins helps combine for other achievement).
When you start everyone should race out and find a flag on the opposite team like any usual game of Capture the Flag (Axis takes British flag) and return it to your teams base (where you spawn).
After a while you will all have learnt the flags locations so you can manage to get 1 or 2 flags each in just 3-5 minutes of playing the game (it ends when one team captures all 5 flags).
After Capturing 25 the achievement will unlock after the game has ended.