On the Kalendáře page, select a calendar you like.
Click Vybrat.
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 Přidat do mých kalendářů.
You will be taken to Moje kalendáře page. On the calendar you just added, click and select Zkopírovat 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