How can I score well in the synoptic essay?

There are two main parts to the synoptic essay, the subject/specification content which will probably make up around 80-90% of your essay, and then the extra content, which is composed of things above the syllabus. For the most part, unless you're already scoring 20-22/25, you don't need to worry about using stuff outside of the syllabus as your time is better spent cementing your subject knowledge - it's very unlikely the examiner will give you the marks for your own knowledge if you're not able to confidently and correctly use the content from the syllabus. I'd recommend constructing 'model paragraphs' which would fit into most essay titles. These would be paragraphs on things like enzymes, ATP, homeostasis, all of which come up year after year. If you prepare a list of 10 paragraphs, in the exam you'll be able to use 4-5 of them, and you'll also know the topics on which your paragraphs are based incredibly well, so it's an excellent revision tool as well as an excellent way to score highly on the synoptic essay question.