From the Booking.com website:
Log in to your Booking.com account at admin.booking.com
Go to Rates & Availability and then to Calendar, or click on Calendar & Prices, depending on your extranet version.
Click on Sync calendars. On the right-hand side, you’ll see the section “Decide what you want to export”, where you can choose whether to export Only reserved dates or Reserved and closed dates.
Once that step is configured, click on Connect another calendar. In the first step, select “Skip this step” to go directly to the calendar export option. You’ll be asked to name the calendar — we recommend using "Check-in Scan".
In the final step, you’ll be able to copy the iCal link and paste it into your Check-in Scan account.
From the Check-in Scan website:
Log in to the web control panel of your account.
Click on the Properties section.
Click on Edit.
Inside the property settings:
Click on the iCal tab.
Simply paste the link extracted from Booking.com and then click on Import iCal.
In a few minutes, your reservations from Booking.com will start syncing in the Reservations tab of Check-in Scan.