To write an introduction about a process diagram, just paraphrase the question statement as usual.
For the overview, here are some general things that you could mention:
- the number of stages in the process
- where / how the process begins and ends
- the items (e.g. materials) involved in the process
Let's return to the 'cement' task and write the introduction and overview.
The diagrams below show the stages and equipment used in the cement-making process, and how cement is used to produce concrete for building purposes.
Introduction and overview:
The first diagram illustrates the process of cement manufacture, and the second diagram shows the materials that go into the production of concrete.
It is clear that there are five stages in the production of cement, beginning with the input of raw materials and ending with bags of the finished product. To produce concrete, four different materials are mixed together.