Here are a few brief, but practical tips I can give you.
1)Research about the enlightenment period-what were the key ways of thinking of the time? How were people like? Were there any major events that defined the period? Were there any cultural/religious/social/philosophical/scientific developments that differentiated this period from others-how did people speak (often reading some extracts of classic novels writing about this period would be enormous help) -for this instance-focus research on social ills-but you can also research other things to give yourself a holistic perspective of the period.
2)Then consider what your main characters are, consider what plot development you want to integrate to reflect social ills-any symbolism, extended metaphor that you could use to reflect your research into this period-the key is authenticity-and not necessarily many plot twist-focus on believable plot developments-and plan as much as you can before you start writing-make sure your writing reflects your research!
3)Then just start writing-because it is impossible to conceive a perfect plot by thinking about it forever and not starting-so be brave, start writing and then you can step back critically and ask for advice/critique to fix it up and improve upon it
Best of luck for your creative writing ventures
Sometimes reading some past extension II english works in writers showcase books-published every year for exemplary English Extension II work in HSC-would give you good insights into what good narrative writing looks like