Squeeze some lemon juice into a cup and then use a cotton bud to write the clue onto a piece of paper. Make sure the paper is not too thick otherwise heating it is difficult. The clue will be slightly visible but not legible. To reveal the clue the hunters must heat the paper slightly (for example, over a hot light bulb or a candle flame).
Try it out yourself a couple of times to ensure you know how much lemon juice is required for each word.
This website gives fuller instructions and explains the science behind it: Science Kids - Invisible Ink with Lemon Juice.
For an alternative to this see our Invisible Black Light Writing clue