Item detection.

So, with command blocks, I've seen that there is a way that you can detect certain items on the ground and make it into some kind of custom item. Well, that's cool and all, but for the purpose of my mod, I want to do that. I could just make it a craft able item, but trust me, in my mod, it doesn't make sense.

So, I want to create it by dropping certain Minecraft items on the ground. I'm pretty certain there is a way to do this with code, but since I'm terrible at coding, can someone help me through it? I really need help with this. Thanks.