First, you go underground with a friend, then you go into your friends secret base and go to their PC. Then, you press the A button in front of it and you steal their flag. A good method for getting higher ranks is to put your bases right beside each other and steal eachothers flags.
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No. however, if you leave your door open and someone finds it, they can come in to steal your flag.
you need to steel a flag
First, you and a friend (it requires two people) must both have made a secret base. Then, you and your friend have to be underground at the same time, playing simultaneously. Then, go to your friend's secret base. Press 'A' while facing your friend's flag, and it will ask you if you want to take it. If you press 'Yes', you will get the flag, and you can run to your secret base to click 'A' on your computer and successfully win 1 game of capture the flag. Your friend can click 'A' on you and take their flag back (basically tagging you), and you have to go back to their secret base and find them and take the flag again. Your flag will change colors the more times you win the Capture the Flag game.
the first star is easy (beat the elite four and champion)the next star can be obtained 4 ways:Winning any Master Rank Super ContestEarn a Platinum Flag in the Underground by getting 50 flag capturesCollect all 493 Pokemon in the national dexDefeat 100 Trainers in a row in the Battle Tower
You get gems by increasing your flag rank in your secret base in the underground. You rank up when you capture other peoples flags.