The template is composed of:
One Epic issue,
Any issues linked to the Epic (relation “Epic link”),
Any sub-tasks in the Epic,
Any sub-tasks in the issues linked to the Epic.
An example of Epic template looks like on the picture below:
To create issues linked to Epic you can use any issues type you have configured in your JIRA instance.
Issues in Epic can be a part of any JIRA projects created in your JIRA instance.
How to create Epic and linked issues you can read on Atlassian Jira Epics Tutorial.
More about sub-tasks you can read here Atlassian support page.