For chem and maths, your best bet is just to do practice papers. Use a textbook and youtube to go over any topics you can't remember anything about, then do the following:
1. Practice paper (maybe do the first 2-3 open book since you haven't revised much yet, then do them under timed exam conditions)
2. Mark using the mark scheme (and make sure to really understand where you've gone wrong)
3. Identify problem topics and talk to teachers or friends, consult youtube/a textbook etc. to make sure you understand the topic
4. Do targeted exam Qs on these topics (preferably under timed exam conditions - 1-1.5 min per mark is a good estimate)
5. Mark using the mark scheme and understand where you've gone wrong
And then repeat until your exams. At first you'll struggle and get low grades, but as you go on you'll be able to answer more of the Qs and understand more of the content so your grades will start to improve. This approach allows you to cover as much of the specification as possible in a short amount of time and do targeted revision on specific topics while practicing your exam technique. Remember it's quality not quantity that matters.
Good luck!