If you're talking about things like Fire Flowers shooting Ice or stuff like that, you'd need to hex the items and put them in their respective folders like any other .Pac hack.
If you're talking about stuff like stacking, that's part of the GCT, which would be found in the "codes" folder on your SD Card. I'm pretty sure there are some GCT editing programs you can find on this site, but I don't use any of them and therefore I can't point you in any direction. Just know that they're here.
If you want to make your own GCTs, you'll need CodeManager (an exe for your computer) and this site here:
http://www.geckocodes.org/There's a Brawl section there as well, but you'll need to chose the codes by which version you have. (NTSC, PAL/KOR, NTSC-J)(North America, Europe, Korea, Japan)