You can add any number of subtasks by visiting the "Checklist" tab for any Task.
Click Add Items to get started.
To the right of each item you'll find controls for managing it:
- Edit the item
- Delete the item
- Re-order the item in the list (via drag and drop)
The bottom of your Task card will keep you informed of the Tasks' checklist item's progress, showing you both the total items, and the number of items completed.