After the events of My Little Pony Equestria Girls: Rainbow Rocks, Sunset Shimmer begins writing friendship reports to Twilight from the Equestria.

My Little Pony Equestria Girls: Rainbow Rocks

Sunset Shimmer: Ever since you started this band, you've been letting little things get to you. I never said anything 'cause I didn't feel like it was my place. Not when I'm so new to this whole "friendship" thing. I still have a lot to learn. But I do know that if you don't work out even the smallest problems right at the start, the magic of friendship can be turned into something else.
Twilight Sparkle: I can't believe all this tension was happening right under my nose and I didn't realize it. I'm supposed to be the one with all the answers. And all I've done since I got here is let you down.
Sunset Shimmer: I don't think anyone is supposed to have all the answers. But you can count on your friends to help you find them.

The friendship lesson for this film is written by Sunset Shimmer.

Dear Princess Twilight,
Missing you already, and I hope you'll be back soon. Things are definitely looking up for me here at Canterlot High. But I know I still have a lot to learn about friendship. Hope you don't mind if I write to you for advice when I need it.
Your friend,
Sunset Shimmer

My Little Pony Equestria Girls: Friendship Games

Sunset Shimmer: This isn't the way! I know you feel powerful right now, like you can have everything you want! I've been where you are, I've made the same mistake you're making! I put on a crown and, just like you, I was overwhelmed by the magic it contained! I thought it could get me everything I wanted!
Midnight Sparkle: Oh, you're wrong. Unlike you, I can have everything I want!
Sunset Shimmer: No, you can't. Even with all that magic and power, you'll still be alone! True magic comes from honesty! Loyalty! Laughter! Generosity! Kindness! I understand you, Twilight, and I want to show you the most important magic of all...
[transformation noises]
Daydream Shimmer: ...the Magic of Friendship!
Daydream Shimmer: Take my hand, Twilight. Let me show you there's another way...just like someone once did for me.
Sci-Twi: I may know about a lot of things, but friendship isn't one really one of them. And I'm definitely not gonna learn more about it by being alone all the time.
Fluttershy: Still no word from Princess Twilight?
Sunset Shimmer: Not yet. But I think I may have figured out how magic works in this world. We pony up when we're showing the truest part of ourselves. I was so busy waiting for someone else to give me the answers that I gave up looking for them myself. I'm sure there will be more magical problems that pop up in this world, but like Applejack said, Princess Twilight has her own problems to worry about in Equestria. We can't expect her to always be around to help us.

The alternate ending seen on the DVD also provides these lessons:

Sunset Shimmer: It's actually kinda nice to know I have someone at CPA I have so much in common with.
Sci-Twi: And it's nice to know that CHS is a place where I can always find the Magic of Friendship.
Sunset Shimmer: I bet you'll be able to find it at Crystal Prep pretty soon, too.
Sunset Shimmer: Actually, I've been thinking about it. I know the magic in Equestria's dependable, but there's something cool about not knowing what it's gonna do next.
Rarity: Oh. Well, that's certainly the case here.
Sunset Shimmer: And after everything we just went through, I feel like not knowing something isn't always so bad. It gives ya something to work for.
Rainbow Dash: So, what're you saying?
Sunset Shimmer: Equestria has a magic all its own. But this place does, too. Someone has to figure out how it works here and it may as well be me, so... I guess I'm saying... I've decided to stay.

My Little Pony Equestria Girls: Legend of Everfree

Sunset Shimmer: Yes, last time I turned into something amazing, but I've let magic turn me into a monster, too. So if anyone understands what you're going through, it's me. I can help you, Twilight. And the rest of our friends can be there for you, too. But not if you run away.
Sunset Shimmer's voice: Twilight, listen to me! You are in charge!
Rarity's voice: You are a light, darling! A force for good!
Rainbow Dash's voice: Yeah! You can kick the darkness's butt!
Pinkie Pie's voice: We're here for you, Twilight!
Applejack's voice: And we'll be here, no matter what!
Fluttershy's voice: We believe in you!
Sunset Shimmer's voice: You... are not... Midnight Sparkle!
Twilight Sparkle: No! I... am... Twilight Sparkle! And the magic I carry inside me is... the Magic of Friendship!
Gloriosa Daisy: I am so sorry. I only wanted this to be the best week Camp Everfree has ever had. Then instead I've made it the worst. Maybe it's for the best that I'm losing the camp to Filthy Rich.
Principal Celestia: No. It's not. This camp has meant so much to so many people, my sister and me included.
Vice Principal Luna: Why do you think we wanted our students to come here?
Twilight Sparkle: We can't let Filthy Rich take this place away! If camp meant so much to you two, maybe it meant as much to the other campers who came here in years past.
Sunset Shimmer: And maybe we can get them to help save it!
Gloriosa Daisy: Thank you. For everything. Oh, if I'd just asked for help in the first place...
Rainbow Dash: Hey, don't sweat it, it's kinda what we do.

IDW comics and chapter books

Equestria Girls Holiday Special

Rarity: After all, you're family! And you know what I love about family? No matter how big your family is... there's always room for more.

My Little Pony Equestria Girls: Rainbow Rocks - The Mane Event

In addition to the lessons from the film, when returning to Equestria, Twilight states something else she has learned.

Rainbow Dash: Tell us everything!
Applejack: Whatja see?
Fluttershy: What did you do?
Rarity: What did you wear?
Pinkie Pie: What did you learn?
Twilight Sparkle: I learned that I still have a lot to learn. I’m the Princess of Friendship now, and lots of ponies are going to be looking to me to solve their problems. But it was foolish of me to think that I should have all the answers. What I do have are good friends who will always be there to help me find them.
Rarity: Well then, I think we should get started right away. You’ve had quite a bit of mail since you’ve been gone.
Pinkie Pie: From Las Pegasus, Baltimare, Fillydelphia…

Sunset Shimmer's Time to Shine

The friendship report for this novel is written by Sunset Shimmer.

Dear Twilight Sparkle,
A lot has changed in my life because, when it comes to friendship, there's a lot to learn and a lot to practice. Every day I discover something new about what it means to be a good friend. I have been wondering, however, about these little rainbows that appear sometimes. Sometimes they happen when we are singing and sometimes they happen when we are having a good time together. What kind of magic are they? Are they the same kind of magic as ponying up? Is there anything I should know about them? Just curious.

Twilight's response:

Dear Sunset Shimmer,
Sometimes the magic really does happen, but you don't have to understand everything. Just be grateful for it, as I'm sure you are.
Your friend,
Twilight Sparkle

Sunset's response:

Still. There's nothing wrong with being curious is there?

Twilight's response:

Careful, Sunset Shimmer! Careful!

Twilight's Sparkly Sleepover Surprise

Sci-Twi: Sunny Flare, you don't have to be like this. You don't have to be mean. You can have friends if you want them.
Sunny Flare: What are you talking about?
Sci-Twi: I've been where you are. I've been a monster, too. I thought winning was more important than friends. But there's nothing more important than friends. Nothing in the whole world.
Sunny Flare: I have friends.
Sugarcoat: Kind of.
Sour Sweet: When you're nice.
Sunny Flare: Twilight's the one who's a real monster. Not me.
Sunset Shimmer: We can all turn into monsters sometimes. But the important thing is to know when you do.
Sci-Twi: And there's something even more important than that. We're all going to forgive you when you do. That's what these girls from Canterlot High have taught me. We all make mistakes some times.