Combined sciences advice Watch
Once you've built up the fundamental concepts of the ides you need to know, you may proceed to read outside of the specification (try not to go too in detail, for example, ending up learning the quantum mechanical model of the atom for no reason). That's it! In my honest opinion, as long as the basic notions from the specification are clear and you have a deep conceptual understanding of them, getting an 8 or even a 9 won't be that hard.
Then again, all of this is coming from a year 10 student; take that last comment lightly.
To get an 8 or 9 I'd suggest using this summer holidays to go over year 10 topics you've covered and highlight the parts that you "fully understand", "sort of know" and "have no clue" - maybe a topics list would be great and you can Red, Amber, Green them.
Then obviously you want to try and learn them again and some great tools are:
> Bitesize GCSE
> Combined Science AQA (9-1) Student Book (my exam board was AQA)
> Exam Practice Papers (CGP)
For me personally, I prefer to make flashcards and posters because I enjoyed making them and it's kind of nice to look at with all the bright colours, so it was easier for all of it to get stuck in my head (especially for equations!). I simply used my class notes for this rather than doing anything online, because the "different exam board things" used to confuse me.
When you go back to school after summer, you'd then know what topics you couldn't understand and also what you could. Consult with your teacher and tell them about it - and also tell them you're aiming for an 8 or 9 (because then they'll push you more in lessons). When you don't understand some things in a lesson do not be afraid to ask for help - if you don't prefer doing so in the lesson then maybe after the lesson. It's better looking "stupid" for a second than being "stupid" forever.
Also, don't beat yourself up about it too much. When you're relaxed it's easier to learn. Good luck for your GCSEs!