First off, you will need at least 6 players to be able to start a ranked match of any kind. I found the easiest way to boost this was on Trenches, 5 flags, 30 minutes
The 30 minutes will make it easier to get
Once you've decided who is saving the flags, send them to the roof of the middle building. The correct building has a plank leading up to the roof on 2 sides, and 1 flag for each team is on top. Have all players from the other team continuously grab the closest flags and run up the plank to the saver's position. Have the saver kill them, and save the flag just by running over it.
The kills you earn while doing this will also give good progress towards