Rent calendar is a software that helps manage daily rentals, and here is a general overview of how it works:

1. Download the rent calendar app from the iOS App Store, Google Play Store, or open panel.rent-calendar.com
2. Register for the software using a correct email address, as login details will be sent to the email.
3. Add a portfolio, which can be a villa, apartment, room, boat, car, or anything that can be rented out.
4. Once you have added your portfolio, you can add reservations to your calendar.
5. You can synchronize your calendar with other websites like Airbnb, Booking.com, and VRBO using an iCal link.

There is also a video and article available to help with the synchronization process.