It seems like I saw somewhere that you add tylose to fondant to make it harden.

If yes, how much do I add? If not, what should I add instead?

It depends on the fondant (but I prefer Gum trag because it's a natural ingrediant) but roughly a teasspoon - tablespoon per pound. Make sure you let it rest while sealed for at least an hour before using.

add a little tylose. That usually helps