Main Menu -> Scrabble Flash -> Scrabble Flash.
Assuming that you know how to spell more than 50 words, this one will come naturally as you play the game. Remember that if you play a long word, you can usually replay it for a shorter word.
IE: PASTE can become PAST.
IE: YEARS can become YEAR. (etc)
The game will make a unusual alert-type sound when it sees you've made a word that hasn't been played within that round.