Xjyuk

I want to describe my first person perspective character who's narrating without doing the Wattpad "mirror scene." How would I go about this?

Have other people comment on their appearance.

If they are an underdog, a random stranger could be judgmental either to be cruel or out of ignorance. You can easily get broad strokes from an epithet: race, hair color, body type, gender appearance. It could also be a backhanded compliment (assuming they are too young to drink, for example).

They might have an insecure parent or ambitious spouse who attempts to coach them how to dress or groom. It would serve dual purpose because we'd be learning what the other character values at the same time.

A friend could compare themselves as different "types", which the MC might not really agree with. Similarly, a relative could compare them to an older relative in a neutral matter-of-fact way. Again we're learning about 2 people at once, or the truth is filtered through another character's eyes.

As naturally as possible, which means it depends entirely on the character's personality and the actions they are doing.

One thing to keep in mind is to refer only the most important physical features. Here are a few examples.

1. (partially inspired by my brother)

I was already late and wondered if I should spend the time to put on my lenses. I still wasn't used to putting them fast, but ended up deciding against the glasses. As usual, it didn't fir properly at the first attempt, but when I had finally finished I looked at myself in the mirror. I could see how the dark brown of my eyes became slightly different. My sister insisted she could see no difference, but I was sure the lenses made the dark duller. It was depressing, really. Mousy lackluster brown hair matching now dull brown eyes. Oh, well. Anything was better than glasses.

2. (also partially inspired by my brother)

I put on my jeans and decided once more I needed to start exercising. A few sit ups every morning would surely put an end to that flacid abdominal folds every time I sat down. Or I could get a new pair of a different size. Only then my ass - which was my best asset - wouldn't be as visible. Besides, I doubt my mum would let me get another pair. She'd just bought me a pair of Nikes and she'd complained about how expensive all my stuff was.

I put on a T-shirt and grabbed my hair gel. I had had my hair cut with the now fashionable stripes on the sides, but the short bangs in the front insisted in going all spiky. I applied a bit of gel and carefully combed it with my hands till it looked right.

"John! You're going to lose the school bus!"

3. (inspired by a small cousin of mine who used to enjoy praising the eyes of everyone)

"Look at me, aunt Ginny. Oh, your eyes are so, so pretty! I wish my eyes were blue like yours."

"Don't be silly, Nina. Your eyes are pretty too."

The girl embraced my waist fretfully and I sat her up on my lap, letting her rest her head on my breasts. At least she didn't say they were better than all her big pillows anymore. The firt time she'd said that, I'd been at a wedding and I could have died of embarrassment.

"When I grow up, I want to have blonde curly hair just like you."

I laughed! Those curls were the bane of my morning routine.

"How about I get you a new hair pin, honey? Something with silvery glitter to shine in your hair like a princess's?"

"Yay!" She clapped, sliding off my lap. "Mummy, mummy! Aunt Gina is going to buy me a princess hair pin!"

As per the examples, make the description organic and natural, both for the scene and for the character's personality.

This depends on whether you're using past-tense or present-tense narration: whether the narrator is looking back at things that have already occurred, or describing events as they occur.

If you're using past-tense narration, the narrator can always describe themselves while setting the scene, just as a normal third-person narrator would do. One of my novels uses first-person past-tense narration, and I set up her description of herself like so:

Apart from the [giant robot] in my backyard, my life really wasn't that interesting up until my first battle. But I suppose I should talk a little about it anyway.

She then provides a paragraph of personal information, a brief physical description, then segues straight into the actual storyline.

If you're using present-tense narration, it's going to look a bit odd if the character suddenly pauses the action for a couple of paragraphs to describe themselves. The best approach may be just to drop bits and pieces here and there when they become relevant, either through the narrator themselves or through other characters pointing them out.

+1 wetcircuit; my initial thought as well (have other people comment on their appearance). So, something different:

You don't need a mirror for the MC to comment on themselves, particularly with friends. The key here is that if her/his looks matter to the story, there are consequences (and if there are no consequences, the looks don't matter).

And those consequences, in their personality and interactions and thoughts and relationships and even behavior, can reveal the details of their looks. If the character cannot reach something on a high shelf in a grocery store, she's short, she asks a tall guy for help. Vice versa: If you want to tell us Mike is tall, have him pass by a short girl (or grandmother if you want to ensure no romantic interest) that asks him for help reaching something; and he does her a favor.

If you don't tell us anything, we'll presume your MC is average, or above average, since we tend to project our own desires on them. Figure out how the unusual features have impact on their life, or make something up so they do.

e.g. for a female MC with green eyes (her unusual feature):

Karen said, "Guess what? On my shift I saw a little girl with exactly the same green eyes as you! Isn't that weird? Maybe you have a half-sister out there!"

I said, "Well, since they are my Mom's eyes, it would mean she had another baby after me and gave it up. I think I would've have noticed!"

"Oh you're so literal. Maybe you had a baby and didn't notice!"

"Sure, that's it. When I was twelve. I felt really weird one day, must've been then."

e.g. for a female MC that is plain:

"I'm not like you, Karen, if you ask him to dance, he'll be happy. For me, there's a good chance he'll say no thanks, and you know Karen, right? Does she like me?"

"Then tell him I don't, because he's a jerk!"

"That's not exactly the outcome I'd be hoping for."

Karen rolled her eyes. "So if you think Jay is a jerk, why do you like him?"

"He is a such jerk! But remember Mike used to get grabby with Linda, all the time? You know why he quit? Jay pushed him off. And Mike was pissed off, and Jay was like, c'mon pussy! And ever since then, I can't get him out of my head."

Karen frowned. "Well now I like him!"

"Great, you two will be so wonderful together."

Karen laughed. "How about I will like him for you. We have to get him to know you, somehow. Like work on a project together."

I make these examples purposely long to demonstrate a technique; you find a way for the trait to have some consequence, minor or major, but you bury the reveal of the trait in a scene that isn't all about the trait. It comes out in conversation, or personal thoughts, or the reactions of others. It is surrounded with other information. It is conveyed in character. You can do this with multiple scenes for multiple traits (but most people don't have too many unusual traits). The reader will still get it, but you haven't pulled out the "mirror inventory" scene.

+1 to all.

Have your viewpoint character reflect on what they think of their physical attributes. (If this suggestion is in the comments above, I missed it...)

He ran into the Marshalls, headed straight for the clearance rack and
started rifling. God what he'd give to be double-X large right about
now.

One shirt--bright green with stripes--was small enough that it
might fit. He grabbed it, threw his last fiver at the clerk and ran
back out.

^^ That sort of thing.