It's Simple. Keenly analyze the letters you already have on your board (if any) demand for a vowel or a consonant and complete the board to form an English word using the vowels or consonants sent to you.

Be rest assured The algorithm responsible for generating the letters you receive takes into account what you already have on board and sends only letters which can be used to complete the board to create an English word...
Unless of course if the current letters on your board can't be used to create an English word.