When you add a 'Sub-Process (collapsed)' element to your diagram, a blue arrow appears in the lower right corner of the element.
Click blue arrow to open a blank canvas where you can create the detailed subprocess.
After adding at least one element to the subprocess diagram, the arrow appears on the main diagram. Clicking this blue arrow in the main process takes you to the subprocess diagram.
Additionally, a breadcrumb trail is automatically generated to help you move between different subprocesses in a sequence.
Until you provide a name for the subprocess, a random name is assigned in the breadcrumbs, but the diagram name remains empty. You can create an unlimited number of subprocesses in a sequence.