With practice! Or just think of something funny next time if you have to get yourself to smile. [Although that's kind of hard without memories...But that's something easy to fix:]

Think of funny faces, like this-- [Yuya rolls his eyes and fans his hands in hands in front of his face while wiggling his fingers.] Bwuh-wub-wub~

[Even if it's meant for Yuto, the baby seems to be getting a kick out of it too with all his giggling.]