Create an Appointment Reminder account if don't already have one.
Go to Calendar > Add Calendar > Zoho.
You will need to enter your Zoho Username and Password. Its likely you will need to create a Zoho "app-specific" password first. Please go to https://help.zoho.com/portal/en/kb/bigin/channels/email/articles/generate-an-app-specific-password#To_generate_app_specific_password_for_Zoho_Mail to view instructions on how to do this.
Once your username/password has been validated you will see all the Zoho calendars you have access to. Are some missing? Follow these steps.
Click Settings (in Zoho)
Tick the calendars you want to enable for syncing.
Go back to Appointment Reminder and go to Calendar > Add Calendar > Zoho again.
Once you have connected your calendar it will sync to periodically to Appointment Reminder. (Normally every hour).
If you have multiple calendars repeat the same process.
Once connected you might want to go to Calendar > Edit Calendars > Edit and change any settings you need.