On the Kalender page, select a calendar you like and click Abonnieren.

If you are not logged in to WebCal.Guru, the button will be labeled Download. You will not be able to modify subscription settings and you will have significantly smaller selection of calendars. In this case, jump directly to Step 4.

Adjust the subscription settings if needed and click Zu meinen Kalendern hinzufügen.

You will be taken to Meine Kalender page. On the calendar you just added, click Download.

Your browser will ask for your permission to open your calendar application. Click Allow to send the calendar data to your calendar for processing. From now on your calendar application handles the subscription.