Play locally and select Play Custom from the Scrabble menu. Choose Hot Letters as the Game Mode; start the game up. In order to get your letters "on fire" you will need to hold down the
until the bag starts to catch fire; do this when it's your turn to grab letters.
When the bag has flames on it, quickly let go of the
to Deal All. Your letters should be on fire, and this is the state the letters must remain when you create a word. The letters must have flames on them when you press
to confirm your word (There is a difference between "heated up" and "on fire." "On fire" letters will have flames on them, "heated up" letters will just seem like they're gleaming.)
This must be done in one game and the entire word does not have to be all "on fire;" only one or more letters of the word do. I would suggest using a lot of 2-3 letter words when going for this.