Sign in to your account and manage your profile via this link: https://business.google.com
Click "Bookings" and then "Add a link":
Add your booking link (e.g. yourshop.nearcut.com/book) and click "Save":
Note: You can find your booking link in Settings > My Business > Share:
Click the clipboard icon to copy the link and then paste it into the Booking URL field as above:
When you're done, the appointments button will show on your Google Business profile:
Note: The update can take around 24 hours to apply, so you might not see the change take effect immediately.