How do I create a Sub-task?

Create a new Task or open an existing task on your My Action Items list.
Click the teal New Sub-task button in the middle of the task detail page.
Fill in the sub-task detail page just like a standard task.
You can assign a sub-task to yourself or someone else. If you assign to someone else, make sure to fill in the task description as much as possible to reduce any questions or confusion on the Assignee's part.
When done, click Save in the upper right hand corner of the sub-task detail page.
Your original task that was on the My Action Items list, has now turned into a Project and can be found on the My Projects list.
NOTE: Sub-tasks that are assigned to you are still found on your My Action Items list because they require an action from you.
Click on My Projects to view the parent task.
Click on the project name to open the task detail screen and see the sub-task(s) listed near the bottom of the page.
Click Close.

Did you know that creating a sub-task turns a Task into a Project? Cool, right?