On the Kalenders page, select a calendar you like.
Click Selecteren.
If you are not logged in to WebCal.Guru, 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 Aan Mijn kalenders toevoegen.
You will be taken to Mijn kalenders page. On the calendar you just added, click and select Adres naar klembord kopiëren.
Log in to Yahoo! Mail Calendar.
Move the pointer to Others in the left, open menu and select Follow Other Calendars.
Go to the Calendar URL field and paste the clipboard contents there. The result will look something like https://www.webcal.guru/....
Click Save.
Feedback confirms the result.
Updated 8 Feb 2023