you have to go underground with your friends and you go into each others secret base and take there flag and run back to your base with it.
bring the flag to your base put it on your PC get out of base go up.
You have to go to the underground with a friend. Then go to his base and go to his computer, press a, then you will have a flag, and then you go to your own base and go to your own pc, and then you have it.
you need to steel a flag
If you mean in the underground, take it back to your base and go to the computer, you will now have 1 flag and if you get more you can destroy the boulders in your base and add more items
you cant't neccissarily battle, but on Pokemon Diamond and Pearl if you and your friend are underground you can break into their base, steal their flag, and run it back to your base and store it on your computer (meeting people underground will get you a chance of catching spiritomb). Its Pokemons version of Capture the Flag.
You get gems by increasing your flag rank in your secret base in the underground. You rank up when you capture other peoples flags.
As the underground man said, I'm a spelunking MASTER! So, I have your answer.1. If you have a friend that has Platinum, Pearl or Diamond, go underground with them and find their secret base!*Or if you have two DS's and tow game's, put them in the separate DS systems and steal a flag from your self, lol2. When you do, go to their PC and when it asks if you want to take their flag, click "yes"3. Once you have their flag, race back to your secret base and register it in your PC4. If you want another prize, go back above ground and talk to the underground man, and he'll give you your next prize for getting a flag
In your underground secret base, right? You'll have to capture at least one flag underground before you can delete them. Just keep capturing your friends flags and eventually you will have much more room for goods in your base.
you need to get flags from plauing with your friends underground. the flags will be in there base so if you find it take the flag and run
You need to steal the flag out of your friend's Secret Base underground, then go up to the PC.
Talk to the underground man after completing his tutorial and he'll tell you he'll let you move boulders if you capture a flag. go into the underground and get a friend to go on his version and use the DS wireless link. find you're friend's secret base and talk to his PC it'll ask you if you want to steal his flag. take it and go to you're base. Talk to your PC and it'll register. You can now have 12 items in your base and remove 4-5 boulders. Hope it helped you.
To take flags underground, there must be someone else within wireless range. Hunt down their Secret Base, go to their PC, and take the flag. Run back to your base and go to your PC to register the flag.