Good luck for your endeavours - admire your perseverance in seeing this through.
Some of my tips that worked for me - however you do have to note that we all took our exams n years ago, and what worked then might not work now. So take these advice with a little caution...
1) For me, I'll typically write notes after reviewing a chapter. Helps me refresh/conclude my understanding of a topic, and helps for revision as well.
2) There's always a temptation to gloss over things, or memorize without understanding - don't do that. As one of my teachers used to advise me, "that lack of understanding (on a particular point) will come back to haunt you.". Of course, there's always some form of "hardcode memorization" required (e.g. maths formulas, etc).
3) What worked for me is to attempt as much O Level TYS as possible - helps reinforce your understanding, helps you get used to the format of the questions, and exposes you to potential weak areas (that you can reinforce on later).
4) Set a study plan - write down all your topics that you need to cover, and set a schedule. That way, things become more managable, and you know if you're on schedule (or not). Don't cram last min, as it'll backfire on you.
5) Last but not least, persevere - there will be some tough times where you'll want to pull your hair out, but you'll get there (or understand) eventually. Don't give up. Form study groups (with equally persevering friends though!) to help reinforce each other as you all go along.
Hope this helps, and good luck!