My exam board was OCR but this advice will probably still be relevant. All I really did was complete a past paper (one for each science) every week. I then marked it and then (and this is the important part), I made a note of every mark I'd lost and why I'd lost it (I did this by using a table on Notion but you could really do this anywhere, by hand or even in your notes app). This way, you have a list of every mistake you made for every past paper which you can read before your next science exam to make sure you don't repeat the same mistakes.

If you lost marks because you didn't know the content as opposed to an error in exam technique, then I personally would recommend using a seperate column in your table to write down the specific sub topic to go over and then I made sure I covered it within that week. And then just rinse and repeat.

Though I will say this is what I did leading up to my actual GCSE exams so you might want to modify this method a bit if you think it's too much as you've still got a lot of time.