[ASM & Hex] How can I add new held item effects to Pokemon Emerald?

I'm surprised that I haven't been able to find anything resembling a tutorial for this. I have seen some hacks that have new held items and their effects but I have absolutely no idea how you're supposed to do this. Does anyone know to do this or have a link to a tutorial? I want to make items like Life Orb, Expert Belt, etc

There are a few examples in this thread, but most of them are for Fire Red. The easiest way is to use the emerald battle engine upgrade, but if you don't want to use that for whatever reason you'll have to add the effects with ASM or C.

I'm trying to add them with ASM but they just don't seem to be working for whatever reason. I don't want to use the battle engine upgrade because, from my experience, it's quite buggy and has a lot of features I don't really want. If there's some way for me to only use the held item effects from the battle engine upgrade I will 100% do that