Concept of inventory system

So I've been working on an RPG, and I need an inventory system. I've gotten the gameplay concept done: It'll be a "Rule of 99" inventory, where you can have an infinite amount of different items, and items that can be stacked can be stacked in large amounts, usually with a cap of 99 (hence the name "Rule of 99"). The hard part now is that I'm having trouble conceptualizing the overall workings of the system. I've tried several different class based models, and they all ended up pretty convoluted.

So, what I'm asking for is not a script. But I am pretty new, and want to code everything myself to get experience. I was hoping someone has done this before, and could just give me sort of the overall concept to help me get started.