This article assumes you already have a Page set up for your property on Facebook. If you don't, you can follow the instructions here to find out how.
Once you have a Page set up for your property, the process of adding a Book Now button is very simple.
When logged in as Admin/Owner of the page, you should see an "Add button" option. Click this:
On the next page, select "Make a booking with you" as the button type, and click next:
Then, select "Link to website" as the button type:
You will be prompted to enter a booking page URL, which you can get from your Caterbook account. This is located under Settings / Property / Booking Template in the main menu. Click to copy:
And then paste this into the URL field in Facebook, then click Finish. Your Page will reload and will now have a Book Now button, which will take guests directly to a standalone booking page for your property: