With writing discursive writing, I recommend studying perfect examples (20/20) and picking out specific characteristics you enjoy about them to integrate with your writing. Personally, I used to write discursive writing like essays (i.e. you have an idea with introduction, body and conclusion) but I have come to realise that it is much more flexible and you should write whatever comes to mind in discursive writing, which is why I find it better than writing a narrative in exam conditions. The best idea is to pick out a core theme you want to focus on and think about how you would address that within your discursive piece.