Well, here I'll tell you. But this is completely my opinion.
-Um, remember your 3rd grade teacher telling you something about paragraphs? Well do it! Make paragraphs! I don't like it if people don't keep their writing organized (which even I sometimes don't do, so I shouldn't be talking right now). It looks better anyways.
-The title of your piece is also what is shown on display. So, think of a creative title! They catch reader's eyes, and make them want to click on it.
-Alright, I probably should shut up right now, but spelling. No, you don't have to be a kid who has one of those spelling bee trophies. But guess what? There's something called spell-check. Even I make mistakes, so it's totally fine, but really. If I can spot 8 words in one sentence that are mispelled, there's a problem. Same goes to grammar, but I'm a nutcase when it comes to grammar, so I'll end it here.
-'Reader-chan went to the park.' Stop and think about that sentence. Guess what I'm going to say? Yup, you guessed it. BORING. Be exciting! Be in the mood, and try to be descriptive. And as a writer and a fangirl, insert mystery and turn-ons. Also, I'm going to use Gilbert/Prussia as an example, but don't keep calling him 'he', 'him', or 'Gilbert'. There's 'albino', 'awesome Prussian', 'Prussian', and much more. DON'T USE CLICHES! Except a good amount of cheezy-ness would be fine, I guess. *shrugs*
-The kiss. Alright, I'm sure at least one of your Hetalia reader inserts had a kiss. I, Alice Glow, am
-Make it comfortable for you. I mean, I hate writing lemons, it's just really hard, so I usually go with depressing, bitter-sweet, or fluff when I write Hetalia reader inserts. If you like one-shots, fine! Song-fics, totally, as long as you understand the meaning of the song! And long stories, sure, as long as you don't take too long between each chapter, because that makes your readers feel sad. (Easy to say, hard to do. I'm not even supposed to be saying this, I'm completely leaving my 'Russia x Reader' thing alone and out ther for about a month now! I. SHOULD. SHUT. UP.)
-Be confident with your work! But if you get criticized, think of it as a great gift. It helps you improve. (I AM HUNGRY FOR CRITICIZES!!! I NEED THEM TO BE A GOOD WRITER!!!)
-HAVE FUN. Or else you're not fit to be a writer.
Many have argued about the following, and it'd be best if you do not comment or rant, or whatever about this section of advice.
Alright, now you see that place where you can insert a preview picture for your reader insert story? Yeah, that one. Now this picture is pretty important to me. It shows what kind of story it would be about. A humorous picture would let me know I might read a humorous reader insert, a cute picture migh mean fluffy, and a smexy picture might mean I might read a more or less romantic reader insert. Yes, you shouldn't judge a book by its' cover, but humans naturally just do. So your choice of picture can represent what your reader insert is about. If you don't add pictures, you should try to start.
(PLEASE, PLEASE, PLEASE, READ THE FOLLOWING!! I CANNOT STRESS THIS ANY MORE!!)
Here's the thing. Many people say that you should add pictures on your fan-fic if you drew them yourself, or asked the artist themselves if you have permission to use it, and NO OTHER EXCEPTIONS. Not to offend anyone, but I think it's fine, as long as you let your readers know that you don't own the picture, even if you did or didn't ask the artist for permission. (Which, I myself, am trying to put disclamers at the top before my story begins.)
THE END. EPIC FAAAAAAIIIIIILLLLL....