Select the Site Archive tool from Tool Menu in the Administration Workspace.
The term drop-down menu will be populated with the existing terms in your instance of Sakai. Click on the term you wish to archive to select it.
You will see a list of all sites from the selected term. If this list is correct, click Archive these sites to create the site archives.
Note: Depending on the number and size of the sites you have selected for archiving, this process may take a while.