The exact process depends on what application you are using. If you haven't already, you may want to take a look at specific instructions for various platforms.
In general the process is pretty straigthforward. If your calendar application is configured as the default calendar on your computer, no special tricks are needed.
On the カレンダー page, select a calendar you like.
Click 選択.
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 カレンダーに追加する.
You will be taken to マイカレンダー page. On the calendar you just added, click ダウンロード.
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.
Updated 8 Feb 2023