Create A Subproject
Click on the + sign in the upper right corner of the task box.
When you do so, you will create the subproject and be taking into the subproject where you will be presented with a clean canvas where you man plan the subproject.
Projects may contain as many levels of subprojects as you need.
Task With A Subproject
Once you've created tasks in the subproject the + sign will be decorated with a green background indicating that a subproject is present.
You can move into the subproject by clicking on the + sign.
Moving Up A Project Level
If you are in a subproject and would like to move up a level click on the - sign in the upper left corner of the canvas.
Where Am I?
In the bottom left corner you will see a status bar indicating which level of the project you are in, the name of the parent (high level task containing this subproject) and the number of tasks at this level.