Basically you have to write a piece of php code to read your events and write it out in ics format. (Just like writing out a template but a bit different).
WordPress has actions/functions to add a feed and provide a hook that produces the text. You also need to make sure that you adjust the query to list all of the events.
To ensure that your feed is accepted by all the calendar providers, certain things must be in place.
On this website (http://icalevents.com) in the sidebar lower down, there is a list of ics resources info, validators etc