What is meaning of "Decipher the rest of the message by deducing additional words"?
"Deduce" = "guess, and test your guess"

You know where the letters t,h, and e are, so you might find a sequence "the-e". You can guess this will be "there" in the message, and then you have the substitution for r which you can also try, filling in more gaps and giving you essential clues for other words. If anything starts to look wrong, backtrack and revise one of your earlier guesses.

This is a lot like attempting a crossword, but the clues are the context of the words in the message.