You must have an existing Google account if not you need to set up one.
On a browser open https://calendar.google.com then login your credentials
Enter Email the tap Next
Enter Password then tap Next
Tap the + icon on Other Calendars then select From URL
Copy your preferred calendar URL below and enter it into the From URL
Check Make the calendar publicly accessible
Click Add Calendar
Primary school
http://my.cis.edu.sg/calendar/export.php?export=campus&campus=6&token=c495d571975af1bddf148ea5c
Secondary school
http://my.cis.edu.sg/calendar/export.php?export=campus&campus=3&token=c495d571975af1bddf148ea5c
Note: If you have set up already, your Google account on your phone, then no need to do this step. The CIS calendar should already sync into the phone calendar.
Tap on Settings
Tap on Users & accounts
Tap on Add Account
Select Google
Enter Email then tap Next
Enter password then tap Next