To-do items are attached to individual tasks or subtasks. Once the first to-do item is created, the task will show an icon indicating that there are to-do items attached in the sidebar for that task.
To create new to-do items, simply open the task sidebar, type your to-do, and hit enter. To mark complete, check the box to the left of the item. To delete an item, hover over the to-do and click the X on the right side.