Once you have configured Salesforce.com by following the instructions here, you're able to create JIRA Issues from Salesforce.com Cases by clicking on the Save to JIRA button in Salesforce.com

As the button is pressed, a popup window will appear, taking you to the Create Issue from Salesforce.com Case page

This feature requires you to be logged into JIRA. If you're not logged in, it will take you to the login page, and upon successful login, forward you to the *Create Issue from Salesforce.com Case page.

Simply select the Project and Issue Type, and click Next>> . The issue will be created using the data from the Salesforce.com Case, and the comments from the Case will be created also. If you have any Salesforce.com Case Custom fields enabled for the project the issue is in, it too will be set for you.

If the current Salesforce.com Case already has existing JIRA issues that are linked to it, a warning screen will show prior to creating the new JIRA issue