Say hi, everyone!
Step 1: Happiness
This is easily shown with the mouth. It can be in the form of a simple smile, cat smile (shown on chibi Sauwen), or a big smile with an open mouth (as seen on the instructables robot.)
Note: The cat smile can often be a symbol of cunning, like the character may be up to something suspicious since cats are stealthy creatures.
The eyes can be normally open or seemingly closed (somewhat signifying their smile is so big that their eyes squint). In Chibi Sauwen's case, her eyes resemble an upside-down u. In the instructables robot's case, his eyes resemble the >< symbols, signifying that he's shutting his eyes because he is laughing so hard. These eyes may also represent cases in nervousness or excitement, but don't forget to change the mouth!
Finally, the heart in the bubble shows love and affection towards something the character is dealing with.