From the "How to Use the Unfortunate Events Deck" section:
The Keeper may choose to present a player with a card from this deck during play. The player receiving the card should apply its effects to their investigator immediately.
Keepers are advised to pre-select appropriate event cards rather than drawing randomly as each card is unique and should only be used for the proper situation. For example, the Death Scene card should only be given to a player whose investigator is alone and is about to die—the player knows their character has bitten the dust, and so giving them the card isn’t a surprise, rather it is a way for them go out with a bang.
Some cards describe the effects of spells cast against an investigator; in most situations the investigator concerned may be completely unaware that they have been targeted by a spell. In such cases, the Keeper is advised to make opposed rolls secret (to figure if the spell is successful), avoiding arousing too much suspicion in case the spell fails. Note that specific spell names have been left off the cards, allowing the Keeper to apply them as best suits their game.
Other cards provide ‘secret’ notes to players, giving them just enough information on how to act or what they should do next. The Keeper is advised to provide further information to the player concerned as necessary as the game progresses.
Some cards may be used to enhance the story, perhaps advancing the plot or introducing inconveniences, or red herrings. Be wary of putting too many obstacles in your player’s paths as these can both derail the story and frustrate the players.
Keepers retain the right to add or modify the effects of any of the cards, perhaps reducing damage amounts or allowing players further options than those stated.
Use these cards wisely as they can dramatically affect the game.
A few blank cards have been included in this deck, allowing Keepers to devise and create their own unfortunate events to add those here, or to simply use as secret message cards.