It was Hearts and Hooves Day, and Sunset Shimmer was sitting on the floor in her hips, making a card with a heart that was written inside, for my nearest pony I have. I love you. At her side was Garble. He was a newborn dragon cub. When she finished, she looked at the paper and went to Luna’s room. When she reached the door, she set the paper on the floor, knocked on the door, and hurried off to the bedroom. When the door opened, Luna saw a card on the floor. Smiling, she took the card with her magic, read it, and thought, wonder who wrote it? Was it Sunset Shimmer?
Luna entered the room to read. When she opened the card she smiled at what was written, to my nearest pony that I have. I love you.
Yes. It was Sunset Shimmer.
Luna then had an idea. She took out a paper and colored pencils she had in her room.
Sunset Shimmer was in her room, walking in circles, did Luna like my card? What’s was on my mind? It is obvious that she is the princess of Equestria and will not care for that.
Suddenly, Sunset Shimmer heard a knock on the door. She opened the door and saw a card on the floor. She picked up her magic and opened the card and smiled at what was written, you’re also my closest pony I’ve ever had. I love you too. Sunset Shimmer hugged the card. What she didn’t know was that Luna watched Sunset Shimmer from a distance with a smile.
The spelling in this story was pleasingly proper. I didn't find a single misspelling or typo, though some words were mistakenly capitalized and others were used instead of the correct ones, along with a few words missing altogether. The tenses were respected as well, with only a few exceptions.
On the other hand, the punctuation was more miss than hit and the formatting of the paragraphs needs fine-tuning. The names of the characters were repeated much more often than necessary as well, something which using more pronouns would help.
On that note, I appreciate the absolute absence of constructs like "the unicorn," "the lunar princess," "Luna's student" and the like.
The issues I mentioned are widespread throughout the story, so I'll correct only one part of it as an example.
A little polish;
There are more things to improve, but I'm only focusing on formatting, so to speak, and punctuation for this example.
But a story isn't only how it's written, it's also what is written.
I liked the little shared moments between them. Sunset's simple "if I trust you, why not trust [yourself]?" was precious, but there was another I liked even more: her giving Luna a card on Hearts and Hooves Day because she's the person closest to her heart, only to get worried that she simply wouldn't care, before Luna herself dispels these worries with a card of her own.