Here is how Flash Fire steals the crown and entering the portal goes in Code Red's Adventures of My Little Pony Equestria Girls.
(we see a guard patrolling the halls)
(we then see keyhole flash and a door open)
(we see a cloaked figure sneak past the guard to Twilight's room)
(We see the figure take the crown and replace it)
Spike the Dragon:(snores)
Flash Fire:(trips over his tail) Ugh!
Spike the Dragon: Huh? What?
Twilight Sparkle:(yawns and sees her crown and Flash Fire) My crown! He's got my crown! Stop! Thief! He's stolen my crown!
Rest of the main cast: (Gasps)
Twilight Sparkle: Stop!(teleports in front of him)
Flash Fire:(teleports behind her leaving his cloak)
Twilight Sparkle:(tackles him)(grunts)
(we see the crown go through a mirror)
Mane 6: (Gasps)
Twilight Sparkle: What did you do with my crown?
Flash Fire: Sorry it had to be this way... Princess.(jumps into the mirror)
Fluttershy: Who was that?
(The next morning)
Princess Celestia: Flash Fire is someone that an old acquaintance warned me about. He claimed to be his former teacher until he turned to the Dark Side.
Spike the Dragon: He tried to replace Twilight's crown with this one.
(He shows her the fake)
Princess Celestia: Perhaps Flash Fire thought you wouldn't notice right away that this was not yours. By the time you found out, it would be too late to go after your crown and Element of Harmony.
Twilight Sparkle: But I don't understand. Where did he go? Where did he take my crown?
Princess Celestia: You'll soon know more about this place than even I do.
(We see them in the room with the mirror)
Princess Luna: This is no ordinary mirror. It is a gateway to another world. A gateway that opens every thirty moons.
Pinkie Pie: Sparkly!(puts her hoof on it)
Princess Luna:(rolls her eyes and pulls Pinkie away from the mirror with her magic)
Princess Luna: It was always kept in the throne room of the Canterlot Throne Room. But when Princess Cadence took over the Crystal Empire, we sent it here for her to watch over.
Princess Cadance:Twilight, you must go into this other world and retrieve your crown. Without it, Equestria will be left defenseless.
Princess Celestia: I had never imagined that Flash Fire would come to Equestria. Like Changelings, he can form. But unlike them, he can copy another's memory, and bypass anything, fooling anyone.
Princess Luna: Your crown does not belong in the place Flash Fire evidently calls home. And in his possession, your Element of Harmony will no doubt be used to bring harm upon the inhabitants of this world. They will not have the power to defend themselves.
Princess Celestia: Twilight, you understand the importance of your task?
Twilight Sparkle: Of course.
Princess Celestia: Good. Then you must go at once.
(Rainbow Dash blocks her)
Rainbow Dash: Whoa, whoa, whoa. She's going, we're going with her.
Rainbow Dash: Right girls?
Pinkie Pie: Ohhh! I'm so 'nervexcited'!
Applejack: You do realize that's not a real word right?
Princess Celestia: I'm afraid I can't let you go.
Rainbow Dash: What?! Why not?!
Princess Celestia: Sending all of you could upset the balance of this alternate world, creating havoc that would make it impossible for Twilight to return. This is something Princess Twilight must do alone.
Princess Luna: Time is of the essence. On the third day, when the moon is at its peak in the night sky, the portal will close. And once it does, it will be another thirty moons before you can return.
Twilight Sparkle:(jumps into the portal)
Spike the Dragon:(follows after her)
Mane 6: Spike, NO!
Twilight Sparkle: Whoa! Aaah!
Spike the Dragon:(yelps)