On the Kalendáre page, select a calendar you like.
Click Vybrať.
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 Pridať do mojich kalendárov.
You will be taken to Moje kalendáre page. On the calendar you just added, click and select Skopírovať adresu do schránky.
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