When labelling a diagram, the best approach is to look at the diagram first, noting key ideas and any labels or subheadings. This will help you to identify keywords which will help you scan for the relevant section of the passage. Once you have found the section that you think contains the answer, look for references to any of the other headings on the diagram. This will confirm you are in the right place and also help you follow how the diagram is being described. You will then have to read in detail to make sure your answer is correct.