Invention has a lot to it, but the basics are easy to grasp. There's actually an in game tutorial you should do if you haven't already. The invention guild is in a cave north of falador.
You can add an augmentor to t70+ tools and weapons. Augmented items will "level up" as you use them.
When the item reaches a high enough level after using it for a while, you can either siphon it, or disassemble it. This is what actually gives the invention XP. Disassembling will destroy the item. Siphoning lets you keep the item, but you get less XP.
When starting out, you can only disassemble augmented items, and you'll only be able to level them up to item level 5. But later, you'll mostly be siphoning items, and you'll want to be doing that at item level 12. Any higher than that takes forever and doesn't give any extra XP.
The purpose of invention is that you can add gizmos to your augmented items to make them more effective. You can honestly learn this part later once you're more comfortable with the basics of levelling up invention.
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