Configure trusted apps or OAuth for both the incoming and outgoing authentication for your application link.
Add two-way entity links for each of the entities that you want to link between. For example, if you set up an entity link between a JIRA project and a Confluence space, you will be able to use simple textual references to refer to issues in the JIRA project from the Confluence space (e.g. [JRA-1234]). You will also be able to use simple textual references to refer to pages in the Confluence space from the JIRA project, (e.g. [Test Page]).