Syncing Multiple Google Calendars with BlackBerry 10 devices (Z10 and Q10)

Lots of people, including me have been having a problem getting multiple google calendars to show on the Z10 or Q10. This is the method which finally worked for me.

I tried various sync programs on my device, adding and deleting gmail account on my z10, trying activesync, Caldav etc and nothing worked.

The method which finally worked was to set up gmail as Caldav and then to go to https://www.google.com/calendar/syncselect and select all my calendars.