You can't make your own flags because flags are actually bits of memory in GBA's ram. Basically, someone has probably told you data consists of 0's and 1's. Well, when a flag is unset, a certain part of memory is 0 and when that flag is set, that part of memory turns into 1. Where there is free space in ram memory, lots of 0's (in hexadecimal, shown as 00), you can make your flags set bits (value of bit - 0/1) in there.
This probably felt a bit confusing and I know I wouldn't have understood a thing about it 2 years ago myself but still, this is how it goes. There are ought to be some unused flag bits around which you can use for unused ram memory (as explained above, contains lots of 00's of which values do not change during the gameplay). I believe there is a list of flag bits you can safely use in Research & Development section but that's only a belief (as I've seen a thread related to flags not a long time ago).