First you need the URL’s of your calendar.
The easiest way to obtain these is through our web-calendar: https://webcal.soverin.net
When you hover over a calendar (on the left side) you will see a cogwheel icon:
When you click the icon, you will see a new screen with a URL for CalDav clients:
When you copy this URL and use this in Thunderbird/Lightning it should work just fine.