We can start in one of several places with inventing items, including how we intend them to be used and how we want characters to think of them. Is it a plot coupon they need to acquire to accomplish goals? If so, decide why they can’t reach those goals without it and how they’ll use it to be successful; it helps to think of what can go wrong, too, because tasks that go as planned offer less drama. Its function determines properties, which can then help with form. Origin is optional but adds a good detail that can also determine where this item is now and why, a little history adding depth. A final decision can then be which one of our characters has the skills or attributes to use it. But all of these can be done in whatever order works, and starting with an idea is always best, wherever it leads.