ROM Hack How does one create a GBA patch?

[Meteor7]

I understand that there are tools like LunarIPS which can create a patch for a game by selecting the modified and original .gba files, but I'm curious about how to modify the original .gba file in the first place. To be a little more specific, I'm looking to modify Mega Man Battle Network 3 by adding a menu interface on the Navi viewing screen which will let the player freely modify their equipped style.

What I have is:
A basic knowledge of AR codes (thanks to the tutorial here ) as well as the specific address and values which modify Mega Man's style. I've also some light experience sprite hacking pokemon games.

What I'd like to know is:
How to bring up a window (using the same graphics used for text boxes) upon pressing a certain combination of buttons, or by adding another menu option, which will show a picture of a style with a name underneath it. I'd like the player to be able to scroll through the available styles and, upon selecting one, have their current style overwritten.

I don't anticipate this being too ambitious, but as I'm a beginner, I certainly could be in over my head. If these things are too complicated to explain to a noob, could you point me to a tutorial or someplace I could learn the basics of modifying a gba file?

Any help or direction is much appreciated.

 

[The Real Jdbye]

To do that, you'll have to know quite a bit of ARM assembly. Also, some of the stuff on this page will be useful: http://problemkaputt.de/gbatek.htm
It's not something suitable for a beginner to do, unless you want to spend a lot of time learning the ins and outs of the GBA as well as ARM ASM before you can get anything done.

 

[Meteor7]

Woo, that's a heck of a read! Learning all this will be a slow process, but it looks fun. Thanks for the info, Jdbye!