With regards to writing, the most important thing is reading a wide range of texts - both fiction and non-fiction. You've essentially got to write like a published author to be GD in Y6. The best writers I've taught have developed their voice through extensive reading and writing practise. The primary curriculum encourages writing to be far too flowery for the sake of it - a GD writer will be balanced and selective. Read together and have deeper discussions around the text. Highlight the vocabulary, punctuation and the way things are written to make a certain impact. Give her the tools and encourage her to write her own stories/texts.
With regards to reading - obviously the above will be helpful for this too. Again, discussions when you're reading together - what the book is about, what do you think the character is feeling, what could happen next. Look up KS2 or Y3 reading prompts or comprehension questions if needed. Try to make the questioning natural rather than a test, this can take all the fun out of it.
Maths - knowing her times tables inside out will unlock so much of the curriculum. Practise problem solving and reasoning style questions and answers. She will need to have a complete understanding of each learning objective and be able to justify, show and prove her thinking in a number of ways. Consider whether she can explain the concept in different ways, using abstract, pictorial and concrete resources. I like to get my GD children to plan a lesson and teach me a certain concept - this usually highlights the misconceptions I need to teach them to get them to the next level.
Revision books are good but the subject knowledge of the adult helping her will be the most important thing. It's also really important to not push her too far and take the magic out of learning. Always aim to peak curiosity in your approach rather than just asking test-style questions and teaching with memorisation as the goal. I use a lot of 'I wonder what the author is thinking' and 'how can we find this out'. Create a curious learner and set her up with the skills to research herself. Create an environment at home that can help with this too - times table posters in the toilet have been a success in my house.
I'm sure there is more to it but hopefully some of those ideas will help. I want to add that it is also a really amazing achievement to be working at ARE, the curriculum is tough.