"Tap the + icon at the bottom of the main page to add a new task. You can also swipe up to see the Review page and tap the + icon in either the "today" or "plan" views to add a new task.
Please note, you must enter a description for the task to be saved to your to-do list. Tasks are set for "today" by default, but if you would like to set the task to be due on another day, select "tomorrow" or "custom" to set another date. If you're unsure when the task should be due, you can also set it to be done "someday." These tasks will appear at the end of your to-do list for any given plan.