From the wikipedia page: Ghost is a written or spoken word game in which players take turns adding letters to a growing word fragment, trying not to be the one to complete a valid word. Each fragment must be the beginning of a scrabble approved word, where only words longer than three letters count. The player who completes a word loses the round.
Start a round by saying "start". Alexa will add letters to the word fragment, and you will have three seconds to respond. If you would like to hear the word fragment again, say "repeat". If you would like to add a letter, say "my letter is" then your letter.