Here is a link to the BBC website on discursive writing. It gives a reasonable summary of what you can do.
http://www.bbc.co.uk/education/guides/zw8sgk7/revision
Here are some other links to documents on school websites to help you.
http://www.eyemouthhigh.org.uk/sites/default/files/attachments/Introduction%20to%20Discursive%20Writing_0.doc
http://www.google.co.uk/url?sa=t&rct=j&q=&esrc=s&source=web&cd=16&cad=rja&uact=8&ved=0CEEQFjAFOApqFQoTCIezqrS-iskCFQe8FAodKV0ATw&url=http%3A%2F%2Fdallison1983.edublogs.org%2Ffiles%2F2014%2F09%2FNational-5-persuasive-essay-2lxnmpp.ppt&usg=AFQjCNEoqy-vooHY5R-4T9UM00ZqPHQe-w&sig2=qz7O0jW7SQxUUvhpObwXNg
Also, the shortcut below takes you to an SQA PDF, which gives you information, in a grid at the end of the document, about how the discursive essay is marked
http://www.sqa.org.uk/files_ccc/GAInfoNational5EnglishPortfolio.pdf
