Select your plan, and add your high level tasks. Then for each high level task add the sub-tasks required to complete it.
For example if your plan is “Decorate house” your first set of tasks might be “Decorate front room”, “Decorate kitchen”, “Decorate bedroom”.
Your next set would give more detail, for example “Clear walls and floors”, “Fill holes in walls”, “Paint walls”.
Then structure your plan. Select the “Edit” menu and drag the sub tasks under the high level tasks in order. Then exit “Edit” and right-swipe each sub-task in turn. The sub-task will move to the right and the task above will become a parent.