I don't own the Mortal Instrument series. Note that this contains spoiler from COG.
Tell Jace he's bringing sexy back.
Every time you see Simon yell ", hey look its rat boy" and have Jace help.
Yell "shotgun" randomly while Alec and Jace are sparing.
Ask Jace to tell you curse words in other languages. Then record it and put it on repeat in Alec's room.
If he asks why then tell him it's to express your love for Simon.
Ask Jonathan what's it like to go black.
Tell Isabelle it's okay to look like the bride of Frankenstein.
Yell at Simon "why you no die in the sun!"
Tell Alec it's okay that Isabelle can kick your butt.
If Rafeal ever tries to hurt you push him in the sun.
Ask Luke why he smells like dog.
Lock Jace and Simon in a closet for a day.
Wear a shirt that says "Jace x Clary" around Simon.
Wear a shirt that has a photo shopped picture of Simon and Clary kissing around Jace.
Wear a shirt that has a photo shopped picture of Simon and Jace kissing around Clary.
Watch the chaos you cause.
Tell Magnus, Simon skinned Chairmen Meow.
Stand in front of the institute screaming nonsense about shadowhunters though a megaphone for a few hours.
Put Clary's diary under Jace's pillow.
Paint Jace's room pink, and Isabelle's room black.
When they ask tell 'em Alec did it.
Send hate mail to Simon calling him a sparkly freak, and sign Jace's name at the bottom every time.
Ask Jonathan what it feels like to be replaced.
Destroy Jace's room.
Put a bunch of creepy dolls in Alec's room.
Tell Magnus that Jace needs a pair of skinny jeans.
Throw squeaky toys all over Isabelle's floor, and then blame Alec and Jace.
Tell Maia that Simon got Isabelle pregnant.
And of course tell Isabelle Simon got Maia pregnant.
Repeat until it stops being funny.