I assume you've looked at the mark scheme for the past papers? If not, take a look at https://www.gov.uk/government/publications/key-stage-2-tests-2014-level-6-english-grammar-punctuation-and-spelling-test-materials
According to the mark scheme, "[t]he writing is assessed according to three strands, made up of: sentence structure and punctuation (SSP) [6 marks]
, text structure and organisation (TSO) [4 marks]
and appropriacy and vocabulary (AV) [4 marks]
." They go on to provide three sample answers to the extended task, mark each one, and provide an explanation as to why they've awarded the marks they have.
Perhaps not quite the "recipe" you were looking for, but I'd say it's close.