We went through this last year, and achieved excellent results, we got our top school choice.
Start with Bond books 6-7, then 7-8, do not do any higher even though your kid can do it because it will not calibrate to the test. Except maybe for NVR, then you can do 8-9, because they are simply too easy, you might notice your kid getting bored with the easier ones. But the big message here is that the test is firmly in 7-8 territory, even for the top schools.
Do all the Past Papers you can find online, both girls and boys. Then, sign up for ExamPapersPlus for extra tests, it is worth every penny, we found it to be an excellent resource.
Write stories every day. Not once a week. Every day. Try to aim for your kid having a repository of about ten stories that are fairly scripted, that cover pretty much every setting: School Trip, Birthday Party, Dream, One Scary Night, Christmas, Trip to the Zoo, Day at the Beach, Trip to Mars. The kid should be able to modulate the story to fit the story prompt and be able to produce ten to fifteen sentences with sequential and descriptive wording for each.
Most people underestimate the reading comprehension and writing composition portions of the test. We certainly did. By this time last year, we were acing maths, as in 100% for even the most difficult tests, but struggling with consistent performance in reading and writing (it would be anywhere from 60% to 90% depending on the test.) The last three months we worked exclusively on reading and writing to bring test results up to a consistent 80%, but it never got better than that. If you are bilingual (or even worse trilingual like us), stop speaking any language other than English for the six months leading up to the test.