Another with a DD with inattentive ADHD. This will echo what many on here have already said, but medication for her made a big difference. She was high achieving at primary, did well at the start of secondary and in Year 10 hit a brick wall. She started Year 11 behind in a lot of her subjects and was being predicted 5/6s. She started medication in December and I noticed a huge change in her focus. She achieved really good grades, mainly 8/9s and the rest were 7s.
The tips I would share are:
English Literature - I read the books, poems, watched the play and we listened to podcasts (check out GCSE English RevisionPod - brilliant) so that we could discuss the texts together. My DD also looked at examiner reports to get a good idea of what they were looking for. For example, they don't really want students to learn great long quotes, or even to rely too heavily at quotes at all.
Science/Maths - just loads and loads of past papers (Science and Maths tutor, Savemyexams was also good). For Biology AQA, 'Biology with Olivia' was really helpful as she focusses very much on how to write a model answer using the specific terms etc. Olivia is on TikTok and although you probably don't want to encourage TikTok use, her short videos on how to answer 6 markers are brilliant!
For maths my DD also watched lots youtube videos where they take you through an exam paper and found these really helpful. She went from a 4 at the end of Year 10 to an 8 in the actual exam.
For science, don't forget to learn the required practicals as these can account for quite a few marks, and once learnt they are relatively easy. Freesciencelessons on you tube is good for this (and all the other content) as they didn't seem to have covered them all in class, or if they did my DD didn't seem to have a clue what the point of the practical actually was.
Geography - she seemed to mainly use workbooks to practice 6 and 8 marker answers.
So overall, it was an understanding of the sort of questions that would come up and the sort of answers that the examiners are after that got her through. She also made lots of flashcards, mindmaps etc but it was the exam practice that held her attention the most.
We also convinced the school (finally!) to drop a GCSE from 10 to 9 as she did have difficulty in remembering all the content and doing 10 was just too overwhelming, especially as she started the year with shaky foundations.
Good luck!