Indent reminders to turn them into subtasks of other reminders.
Note: You can’t create subtasks in the Today, Scheduled or Flagged Smart Lists, or in custom Smart Lists. You can also show or hide a subtask by clicking the button to the right of the subtask. If you complete, delete or move a parent reminder, its subtasks are also completed, deleted or moved.
Try a Project Management tool like Monday to organise and manage your tasks.