You need a public address in iCal format in order to import a calendar to FishingBooker. Here's how to do it for your Google Calendar:

1. Open Google Calendar on your computer.
2. Click on the Settings icon at the top right.
3. On the left side, under "Settings for my calendars," choose the calendar you wish to share.
4. Scroll down to the Integrate Calendar section
5. Copy the link visible in "Public address in iCal format." 
6. The iCal address only works if the calendar is made public. 
   1. To make it public, scroll to the “Access permissions for events” section and make sure that the “Make available to public” is turned on"

Note that this cannot be done using the Google Calendar app. If you are trying to set this up on your phone, you must use your browser and go to https://calendar.google.com/calendar (and try opening the “Desktop” view).