Leo Zaccari asked: "What is the best way to describe your protagonist in 1st person POV?It always sounds forced and unnatural. What are some good ways to do it and make it sound natural?"
Excellent question. I think there are as many ways to do it as there are writers, and ultimately you will find your own way. I have accidentally stumbled on a couple tricks while writing my first book, and later I have seen other writers employing other tricks and stole those and tried them in my books as well.
There are many other questions hidden in this question, and those are regarding the specific descriptions needed for your particular character or story. Are clothes important? How about skin color, weight, physique? Any specific behaviors? Age? Gender? Ethnicity? Religious beliefs? Or perhaps fantastical elements like green alien wings or dragon teeth or webbed feet? How important are those things? They might be very important or not important at all. First decide how much of it needs to be in the story, then weave in those descriptions as needed (maybe none will be needed at all).
All decided? Good.
Here are 1st person POV protagonist description methods that I have used or have seen others use. Chime in if you have seen or are successfully using any others.
Siren Suicides begins with a scene in a bathroom, so inadvertently I have used the mirror to have the protagonist look into it and describe what she saw. Later, when writing Irkadura, I used it again, twice. I didn't do it at the very beginning, having the character only state her age, but before she did, I mentioned that she goes to school, that way establishing some age parameters and then narrowing them to a specific age and look.
I have had characters reminisce about the past and compare their present selves to their past ones. Like, they would think to themselves, "When I was three, I was so short, and now that I'm sixteen, I'm as tall as a giraffe." Or something. I have also done comparisons where the protagonist wishes to be this and this, but is actually this and this. "Oh, I wish my skull wasn't so elliptical, I wish it was square." Or, "I wish my boobs weren't falling out of my dress cleavage." Or, "I wish my penis..." Okay, this is quickly getting out of hand. Next!
This is the most common method that I've seen. Your characters talk to each other and state their opinions. So someone could say to your protagonist, "Listen. Please don't flaunt your flabby jowls in my face the next time you bend over to insult me." Okay, I have no clue where this little quip came from. Anyway. Have others describe your protagonist, but only when the story demands it, not because you feel you have to do it. This is important, because the reader will sniff out an info dump in no time and stop trusting you and might even set the book aside.
I haven't seen much of it done, but in Irkadura I have Irka present her passport and her disability documents to a character who reads them aloud. You could have something like that, or maybe a photograph, or any other kind of image that someone from your cast of characters describes. It could be any representation of the character's standing in society, which could also aid you in describing that society, if you're creating a world for a sci-fi or a high fantasy story, or maybe it's historical fiction. You can describe the customs of that society and neatly fold in the character description.
5. No description at all.
You know, you don't even need to describe your character beyond some very basic facts, like a young girl, or an old geezer, or a stately matron, or a tanned self-centered prick, or whoever. Many books have but a sketch of the character, and you fill in the rest of the details. I'd say, have one striking feature and leave the rest out. Like a big floppy hat. Or a red bulbous nose. Or a flat ass prone to farts. Or a kid that never sits still. Did you imagine these people I described? Of course you did. See what I mean? Less is more.
One more little thing you can do is give your protagonist a descriptive name that will eliminate the need to describe him/her. I have done this in The Badlings, although it's not 1st person POV, but the names are Bells, Grand, Peacock, and Rusty, so you can imagine them from names alone pretty well (at least I hope you can).
Okay, I'm running thin here. This is the last one I can come up with. Maybe your protagonist has dreams where his/her alter ego visits him/her? I don't know. Maybe it's a drug induced hallucination? A nightmare? A schizophrenic vision? Go wild.
Whatever you decide to do, however you decide to describe your protagonist, remember:
YOU DON'T HAVE TO CONFORM TO ANY NORMS EXCEPT TO HOW YOU WANT TO WRITE.
You can even step out of the 1st person POV and have the author describe the character. You can switch POVs—I did that in Irkadura. You can really do whatever the hell you want, as long as you think it makes the story stronger.