Karkat finds the shipping wall by accident, and Nepeta spends the day explaining all the ships to him. Except one, which is covered in a blanket. Karkat tries to resist his curious nature, but he can't help it. He comes back later and looks behind the blanket. To find Nep's OTP. He knows it's her OTP, because it's rather large and very detailed.
He spends a few days alone, thinking and plotting. Then he goes to visit her, and he tells her he's going to make his own shipping wall. She comes to watch, and he slowly begins. First all his minor ships. She watches until she falls asleep, and then he quickly draws his OTP nice and large. He sits and waits for her to wake up.
He only wishes he could've taken a photo of her face.
Dave did get a photo of her kissing him though.
So that's something.