I'm a big Reddit fan, not the dodgy side, obviously, but the normal, helpful, lovely person side. I'm what would be considered a 'lurker,' i.e., I don't comment, I just read and scream internally. So I thought I would answer some Reddit questions here. In deference to that actual Reddit thread posed by the noble Sir_MaximusTheFirst, I'll also brace myself and post this over there and hope they don't eat me alive.
The Reddit Question:
How to Describe What Your character looks like in First Person POV?
How to describe your characters in first person POV is a question that has plagued writers since the dinosaurs roamed the earth. I've read books that have pulled the old "I glanced at the mirror" move, and I've read books where I never found out what the main character looked like. It's a conundrum that isn't specific to first-person POV but is definitely exacerbated by it. I feel particularly equipped to deal with this question because I LOATHE describing my characters. I have to force myself to do it, and because it doesn't come naturally to me, I have to find workarounds.
Skip ahead:
Have your character look in the mirror and describe what they see:
The primary difficulty with first-person and character descriptions is that the character can't describe what they can't see. It sounds weird if they describe themselves as beautiful with a knowing glint in their eye etc. etc. This leads to boring writing. The character has to stick with "I have brown hair and brown eyes."
That's why the mirror workaround is so popular, popular enough that it's become a cliche. The character can assess themselves. They can notice the smudge of dirt on their forehead or that their blonde hair is in a mess of tangles. It's more interesting. Sure it's considered a cliché, but who cares? If it works in your narrative, then go for it. The only people who will notice are other writers.
Don't bother describing them:
I'll admit not bothering to describe your character is risky, but it can be a breath of fresh air. Some readers will hate it, others won't even notice. I read a book series recently where I only found out anything about the POV characters appearance in the third book. I noticed, but I didn't care, because the book was good. I wasn't reading it for the characters descriptions, I was reading it for the plot. Overly described characters can be dull anyway. Avoid the trap by not participating.
Have another character describe your POV character:
Personally, this is my least favorite methodology, but if you can find a way to naturally introduce your character's wicked green eyes and luxurious locks through dialogue, I salute you. To me dialogue drags things out, and the only way to do this very effectively is to make sure that the their is a secondary purpose to the discussion. If all that your dialogue is accomplishing is to inform the reader that your character has nice hair, skip it.
Sorry, I have more. The problem is that it's often weird for characters to mention specifics while chatting. It's strange for a character to say, "I like that top, it goes great with your brown hair." More natural dialogue would be, "I like that top, it goes with your hair." Do you see the issue? Maybe you don't. Perhaps I've run mad. It's all possible. So, if you choose this method, you will have to work (or be naturally talented) at keeping everything natural, keeping the dialogue interesting, and not bending it to fit your character's tiny eyes and big ears. Or a more realistic version might be, "you can't wear that top, it's brown. Brown! Your hair is brown, you can't have brown on brown. Awful, horrible, take it off." Then you have to have your character change her top, and suddenly it's a whole thing.
Address your character description head-on:
Sometimes, the easiest route is the best. Think horses, not zebras, and all that good stuff. Have your character just flat-out describe themselves, no props, no gimmicks. "I'm 5,6 with brown hair and brown eyes."
If you want to do me a solid, have your first-person character state that they are stunningly beautiful. Then be sure to let me know when you publish because I'm desperate for a main (preferably female) character whose horrednously vain instead of bashfully unaware of how pretty she is.
Use your character's past to inform their appearance:
This is a great method because you can shove some backstory and character development into your description. Your character was a skinny kid who got picked on by classmates until they shot up two feet in a month. His hair was once dirty blonde, but it's darkened since he started spending less time surfing and more time coding. It blends seamlessly and has the benefit of positioning the character description as a secondary motivation for the writing rather than the primary. As character descriptions can be dull, any method that jazzes them up is a boon.
Compare your character's appearance to other characters:
It's not my favorite method, but comparison has its place, and I've seen it done well. You can describe your character through the appearance of those around them. They can have the same blonde hair as their father, their mother's slim build, etc. etc.
Use world-building to describe your character
World-building can be hard to include without resorting to tedious wedges of exposition. But character descriptions can be a great way to scoot the world along. Use your character's physical traits to develop your world. They are skinny because they are part of the lower class and don't have ready access to food. Their hair is red, which is or is not the prevailing fashion. Their hair frizzes in the humid summers but is as sleek as a sleek thing during the snowed-in winters.
Use hobbies and interests to describe your characters:
Athletic builds make more sense on characters who play soccer, swim, or do something, you know, athletic. If you tell me that your character runs every day, I will assume they are slim. If they surf, I'll assume they're tanned. This way, you don't have to spell it out but rely on your reader. While you describe your character's interests, you can transition more seamlessly into their appearance. This will also help build realism and build your character's personality.
Take advantage of reader expectations:
As suggested in the last point, readers are very good at building narratives out of small suggestions. A scowl, a serene expression, a light step, or a heavy plod. These elements are more valuable than brown or black hair. Hair color tells us very little about a character, but how they wear it tells us a lot. Whether it's been bleached by the sun, intricately plaited, or cropped short is more effective than simply telling us that it's blonde and shiny.
Let reality inform your character description:
Often, we want to protect our POV character from seeming vain—which is why it has become annoyingly popular to have a character woefully unaware of how jaw-droppingly gorgeous they are. We write them as moaning about their appearance to convey how good-looking they are through a negative lens. While you can absolutely do what you want when your write, don't do this, I command it.
If your character is genuinely insecure, they'll spend time fixing their appearance. If they don't care about their appearance, they'll likely have dirty faces and tangled hair. Having a character who doesn't care but is somehow stunningly beautiful and well-turned-out is unrealistic. As a result, it's harder to write. Characters don't build muscles without trying; they don't wake up with makeup. Writing unrealistic characters is harder. Writing a character with rippling muscles that they got from wishful thinking doesn't work.
Be vague:
Character description will seldom be the most exciting element of your story, so there is no reason to use up pages rhapsodizing about the ocean blue of your character's eyes. Let the reader know the basics, and they'll fill in the rest to their satisfaction.
First-person is unreliable; use that to your advantage:
There is something fun about how people think about themselves versus how everyone else thinks about them. You can have your first-person character say that everyone calls their eye color hazel, but they think their eyes are green. They can call their hair blonde only to have another character describe it as brown later in the narrative. Arguments ensue, resulting in amusement for all. It's one of the major benefits of first person, so use it.
That's it. Hope this helps. And many thanks to Sir_MaximusTheFirst for the great question.