How to write an essay about a diagram in the IELTS exam writing test part 1?

Reporting diagrams is descriptive primarily but it is done in the logical order or sequence of events rather than changes. a three part essay will be sufficient for this.

This is the outline:

1st paragraph: Introduction - restate the task

2nd paragaph: Details - describe the process in sequence, highlight what is first, next and last. Highlight simultaneous events.

3rd paragraph: Conclusion - Overall trend, present a general description on how the whole process operates.

This is somewhat the same with writing reports with statistical data, the main difference is that  this should be in logical sequence.