To create a template from existing tasks, first view the task (or set of tasks) that you want to create the template from in the Tree View.
If the details for the task are not visible, click the Show Details icon to show the details for that task. You will see a 'Create Template' link at the bottom of the task's information. Click that link and a new task template will be created from that task and any sub-tasks.
Note: task templates must be enabled in your Server Settings.