How Is Descriptive. What Is Mechanics
The first thing you need to understand when it comes to describing your character's magic, how they fight, or how they accomplish actions, is that the how of things is largely descriptive and as such provides a lot more freedom for artistic expression than the what. Whether a character lunges forward and skewers someone for 8 damage, or does a spin move around an opponents attack then slashes for 8 damage has no impact on the fact the target took 8 damage but they paint very different pictures of what happened to let that 8 damage happen.
By the same token, a wizard pointing a finger and launching a small bird made of flame to a point where it then explodes into a fireball, and one that forms a tiny bead in their hand which they pitch like a baseball to that point - where it explodes into a fireball - are both a wizard casting fireball.
What is the mechanic of the game. How is where you can add in RP and those little personal touches to make your character stand out and be more distinct.
How We Fight
In Jet Li's 'The One' (2001) Jet Li plays both protagonist and villain in a movie about a multi-dimensional killer trying to ascend to godhood by killing all the other versions of himself. This comes down to a climactic showdown between the last two versions of the character. Now Jet Li plays both of these characters - they're physically identical because of that - so how do the film makers differentiate them? Well, they act very differently, and that is important but not relevant to action sequences. However, they also fight differently. Now you don't get Jet Li to not have him do martial arts, so the fighting styles have to be visually distinct. Which we see in this scene.
Note how the Li in the room is doing things in circles, reinforced by the large display of circular art behind him. His movements have flair more than force, but from that they also have power - shown in how the candle flickers and goes out from the movements despite not being attacked. Meanwhile, the Li on the roof has compact, direct movements. He goes in a straight line, enforced by the diagonal line he draws across the screen from a top down view.
You can bring this into your own games with your own characters. Does your character dart in and out where they see openings with quick movements and small cuts and stabs that add up over time? Or do they go for the larger power hits, enduring the storm of their enemy's attacks until they can unload? Do they move around with acrobatic flair? Do they stalk their prey across the battlefield? Are they a more dispassionate, technical fighter using mind games and bait and switch tactics, or do they fight with passion in a more direct style of fighting?
Even two people fighting with the exact same weapon can fight very differently, as shown in the anime Samurai Champloo (to a fairly extreme degree) with Mugen and Jin fighting. Mugen is the one lashing out with his feet, spinning and flipping throughout the fight. Jin has the more reserved approach, dashing and sticking to sword based attacks in a more concise and conservative fighting style. They're distinct enough that I only need to describe the fighting style, not the characters, and you'll know who I am talking about.
Magic Can Be The Same
Magic offers even more freedom. Yes, the books may have descriptions for how the spell works, but nothing says it has to be that way. Nothing says your Scorching Ray can't come from your eyes, or that you can't fart obnoxiously to make a stinking cloud. You can describe them as you want, just as long as what you describe makes sense to do the effect you are looking to have.
Be Mindful of Theme, Power Level, and Tone
The places where people get in trouble with this freedom to describe tends to come in three areas. Namely, the person describing something forgets the themes of the game, the power level of what is going on, or the tone of the game.
If someone is running a dark, gritty, and realistic game, having your character doing acrobatic flips with a half-skeletal bottom half and swinging around a sword the size of a tree is going to come as a break of the themes running through the game. What you describe should fit the world and the way the world works.
Tone is very much like theme. But if the game is super serious, grim, and dark, having a character who does everything slapstick and light hearted is going to break that tone. Extreme theme breaks will break tone as well as the thing goes from ignoring theme to almost mocking it. But other things can too, like the above mentioned obnoxious fart, or other toilet type humor. Effectively, if your game is going for a Saturday Morning Cartoon feel, don't bring in the visuals and elements from SAW. At the same time, if your game is going for Game of Thrones, don't bust out the Bugs Bunny antics.
Finally, power level is the last big one. If your attack does 3 points of damage, don't describe it as a world ending spell. Small looking things doing big damage can be neat, but a whole room being bathed in flame because you did a single target spell may be a bit much. Also linked to power level, is don't try to get other mechanical effects out of your spell via description. Yes, your fireball explodes, but don't describe it destroying the armor off a creature or lighting all the candles and torches in the area providing more illumination to help the group out.
Be Respectful And Beware Grand Standing
The final two points to make are to always remember to be respectful, and try to avoid grandstanding.
For grand standing, try to avoid super lengthy descriptions. What counts as lengthy will depend on your table, but ultimately it shouldn't take you longer to describe what you do on your turn than it took to actually resolve it. If people are giving signs of getting bored, or otherwise looking disinterested, then leave it off. It's fine to go a big bigger for big effects (killing a boss, a big spell the first time it is cast, etc) but be mindful of keeping things moving and the flow of the game.
As far as being respectful, remember where your agency and control ends. You can't control other people's characters, and that includes doing things to them. Your game will have some stuff set for how hits in combat work, but don't just assume that you hit a character with an attack if the game has allowed near misses to explain losing HP. Don't narrate characters you don't control. Focus on what you do. Focus only on what you do, it's safer.