Ideally, plug FELIWAY Help! at your holiday destination 1-2 days before your cat arrives.
If this is not possible, plug it in when you arrive and leave your cat in its carrier for 15 minutes near the diffuser. Then open the door of the carrier and let your cat come out when they choose to. Leave the carrier there, close to the diffuser, with the door open as this will provide your cat with a reassuring place to return to during your stay.