I've made plenty of different magical items for various roleplays and solo writing endeavors. My method for making magical things is to answer a few simple questions: what does it do, how does it do it, and what does it look like?
Generally speaking, I like to start out by answering "what does it do?" Is it supposed to shoot giant pillars of flame? Is it supposed to unlock the door to an ancient tomb? Is it supposed to be used in a ceremony to bring the totally-not-Satan of the world into the mortal plane? I figure out what it's actually meant for, why it has reason to exist, and then move on to the details.
Answering "how does it do it?" is reliant on the magic system at work in the world the item is being created for. If all magical power is derived from crystals, then the object in question will need to have crystals embedded in it. If magic all comes from a pool of mana within a mage's body, then it will need to draw on that power. So on and so forth, the gritty mechanics that probably never need to be explained to the reader but are important for the writer to work out so there's no deus ex machina or inconsistency going on.
"What does it look like?" is the really fun question. Once I have the mechanics worked out, it really doesn't matter much what the thing looks like. Is it a key to a tomb that is powered by crystals? Okay, it could look like a gauntlet, or some unnaturally shaped work of stone, or an old and still functioning golem, or the figurehead of a ship, or whatever amuses me at the time. Generally it ought to fit in with the aesthetics of the world, or at least of the culture of whoever made it, but aside from that it's all up to how I feel at the time of creation.
One of my favorite magical creations was in fact a set of items made for an over the top crazy fantasy world that operated with anime-like physics and power level juggling. I started off by deciding I wanted something that would empower the user to unnatural levels, above and beyond the power of almost anyone lacking such items, but customized for each user. Then I worked out how they operated, which was just "magic in this world is a basically omnipotent force, so they're literally made of magic, and they're sentient so they can adapt to the user's needs." I decided that they should be big ole warhammers, and so they were, each one looking different and having a different set of powers. One of them allowed the wielder to sense the crimes of a target and could one hit kill the guilty, another allowed for insanely powerful plant magic abilities, another one was just about raw strength and defensive fortitude and could morph into different shapes to emphasize one side over the other, and so on. None of them were particularly innovative, but with the character power levels jacked up to 11 and all sorts of over the top nonsense going on they ended up being fun elements that gave already powerful characters even more power to be crazy with. For instance, the main protagonist was a shaman type who could eat the flesh of other things to gain their powers for a time, kind of cool but only of middling power levels in this world, but in the final showdown he got the hammer with the raw offense/defense boosting and ate some dragon flesh he could stand against the big bad evil lady and ended up beating her by taking a bite out of her and stealing some of her power. Cannibalism is always a fun way for the hero to win, right? :D